Greatly decrease the size of rustc_driver.so when debuginfo is enabled

- Only add -gz if it's supported
- Don't include extra unnecessary debuginfo when only debuginfo-level=1 is set
- Compress debuginfo sections to reduce the size of debuginfo on disk.

before: 650 MB
line tables only: 335 MB
compressed only: 216 MB
compressed and line tables: 186 MB
no debuginfo at all: 130 MB

I want to investigate why `-C line-tables-only` is still ~tripling the
size of the binary, but this seems like a good improvement in the
meantime.

I've tested that both valgrind and perf can read the debuginfo:
```
([email protected]) ~/rust [08:31:08]
; valgrind $(rustup which rustc --toolchain rust_stage2) --version
==441671== Memcheck, a memory error detector
==441671== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==441671== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==441671== Command: /home/gh-jyn514/.local/lib/rustup/toolchains/rust_stage2/bin/rustc --version
==441671==
rustc 1.70.0-dev
==441671==
==441671== HEAP SUMMARY:
==441671==     in use at exit: 231,289 bytes in 1,874 blocks
==441671==   total heap usage: 2,538 allocs, 664 frees, 486,368 bytes allocated
==441671==
==441671== LEAK SUMMARY:
==441671==    definitely lost: 70,656 bytes in 1 blocks
==441671==    indirectly lost: 0 bytes in 0 blocks
==441671==      possibly lost: 0 bytes in 0 blocks
==441671==    still reachable: 160,633 bytes in 1,873 blocks
==441671==         suppressed: 0 bytes in 0 blocks
==441671== Rerun with --leak-check=full to see details of leaked memory
==441671==
==441671== For lists of detected and suppressed errors, rerun with: -s
==441671== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
; perf record $(rustup which rustc --toolchain rust_stage2) --version
rustc 1.70.0-dev
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.005 MB perf.data (70 samples) ]
; perf report
Samples: 70  of event 'cycles:u', Event count (approx.): 21356967
Overhead  Command  Shared Object                        Symbol
  51.55%  rustc    ld-linux-aarch64.so.1                [.] _dl_lookup_symbol_x
  18.70%  rustc    ld-linux-aarch64.so.1                [.] _dl_relocate_object
  11.95%  rustc    ld-linux-aarch64.so.1                [.] do_lookup_x
   5.55%  rustc    [unknown]                            [k] 0xffffa9ad41cfcfdc
   2.68%  rustc    libc.so.6                            [.] __GI___strlen_asimd
   2.42%  rustc    librustc_driver-1a385c366c35e81a.so  [.] llvm::StringMapImpl::LookupBucketFor
   2.16%  rustc    librustc_driver-1a385c366c35e81a.so  [.] _GLOBAL__sub_I_X86InstructionSelector.cpp
   1.96%  rustc    libstd-990fe978dab76ef3.so           [.] <alloc::vec::Vec<T,A> as core::clone::Clone>::clone
   1.60%  rustc    librustc_driver-1a385c366c35e81a.so  [.] llvm::cl::opt<bool, false, llvm::cl::parser<bool> >::~opt
   1.22%  rustc    ld-linux-aarch64.so.1                [.] strcmp
   0.13%  rustc    ld-linux-aarch64.so.1                [.] stat64
   0.05%  rustc    ld-linux-aarch64.so.1                [.] __minimal_calloc
   0.02%  rustc    ld-linux-aarch64.so.1                [.] __GI___tunables_init
   0.02%  rustc    ld-linux-aarch64.so.1                [.] _dl_start
   0.00%  rustc    [unknown]                            [k] 0xffffa9ad41cfd844
   0.00%  rustc    ld-linux-aarch64.so.1                [.] _start
```

Author: jyn514

src/bootstrap/builder.rs

@@ -1673,7 +1673,15 @@ impl<'a> Builder<'a> {
                 self.config.rust_debuginfo_level_tools
             }
         };
-        cargo.env(profile_var("DEBUG"), debuginfo_level.to_string());
+        if debuginfo_level == 1 {
+            // Use less debuginfo than the default to save on disk space.
+            cargo.env(profile_var("DEBUG"), "line-tables-only");
+        } else {
+            cargo.env(profile_var("DEBUG"), debuginfo_level.to_string());
+        };
+        if self.cc[&target].args().iter().any(|arg| arg == "-gz") {
+            rustflags.arg("-Clink-arg=-gz");
+        }
         cargo.env(
             profile_var("DEBUG_ASSERTIONS"),
             if mode == Mode::Std {

src/bootstrap/cc_detect.rs

@@ -69,6 +69,8 @@ fn new_cc_build(build: &Build, target: TargetSelection) -> cc::Build {
         .opt_level(2)
         .warnings(false)
         .debug(false)
+        // Compress debuginfo
+        .flag_if_supported("-gz")
         .target(&target.triple)
         .host(&build.build.triple);
     match build.crt_static(target) {