target-specification: add option for linker-flavor

In latest rustc，target specification json file
is needed "linker-flavor"
for this issue: rust-lang/rust#40018

Author: orumin

arm-none-eabi.json

@@ -9,6 +9,7 @@
   "relocation-model": "static",
   "archive-format": "gnu",
   "target-env": "gnu",
+  "linker-flavor": "ld",
   "no-compiler-rt": true,
   "linker-is-gnu": true,
   "disable-redzone": true,