Merge #1749: build: Fix warnings in x86_64 assembly check

ab56007 build: Fix warnings in x86_64 assembly check (Hennadii Stepanov)

Pull request description:

  On the master branch @ 10dab90, the x86_64 assembly check in both the Autotools and CMake build systems can fail depending on externally provided flags. For example:
  ```
  $ env CFLAGS="-Wall -Werror" ./configure --with-asm=x86_64
  <snip>
  checking for x86_64 assembly availability... no
  configure: error: x86_64 assembly requested but not available
  ```
  or
  ```
  $ env CFLAGS="-Wall -Werror" cmake -B build -DSECP256K1_ASM=x86_64
  <snip>
  -- Performing Test HAVE_X86_64_ASM
  -- Performing Test HAVE_X86_64_ASM - Failed
  CMake Error at CMakeLists.txt:111 (message):
    x86_64 assembly requested but not available.

  -- Configuring incomplete, errors occurred!
  ```

  The same issue occurs in CI jobs that build on Windows using clang-cl.

  This PR fixes both build systems.

ACKs for top commit:
  real-or-random:
    utACK ab56007
  furszy:
    ACK ab56007

Tree-SHA512: d556b642d58c601e7f027ac54975249e05a8b3927c5efd229be43d264b024d00eab9973193adb52f2f60075fca0571644662d61150a19098820091ced2d56fa0

Author: real-or-random

build-aux/m4/bitcoin_secp.m4

@@ -3,7 +3,7 @@ AC_DEFUN([SECP_X86_64_ASM_CHECK],[
 AC_MSG_CHECKING(for x86_64 assembly availability)
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[
   #include <stdint.h>]],[[
-  uint64_t a = 11, tmp;
+  uint64_t a = 11, tmp = 0;
   __asm__ __volatile__("movq \@S|@0x100000000,%1; mulq %%rsi" : "+a"(a) : "S"(tmp) : "cc", "%rdx");
   ]])], [has_x86_64_asm=yes], [has_x86_64_asm=no])
 AC_MSG_RESULT([$has_x86_64_asm])

cmake/CheckX86_64Assembly.cmake

@@ -4,10 +4,11 @@ function(check_x86_64_assembly)
   check_c_source_compiles("
     #include <stdint.h>
 
-    int main()
+    int main(void)
     {
-      uint64_t a = 11, tmp;
+      uint64_t a = 11, tmp = 0;
       __asm__ __volatile__(\"movq $0x100000000,%1; mulq %%rsi\" : \"+a\"(a) : \"S\"(tmp) : \"cc\", \"%rdx\");
+      return 0;
     }
   " HAVE_X86_64_ASM)
   set(HAVE_X86_64_ASM ${HAVE_X86_64_ASM} PARENT_SCOPE)