build: Fix warnings in x86_64 assembly check #1749

1. 
   hebasto commented at 9:59 pm on September 17, 2025: member

   On the master branch @ 10dab907e73ba984deb813e20f6e3fc9ec359a17, the x86_64 assembly check in both the Autotools and CMake build systems can fail depending on externally provided flags. For example:

   0$ env CFLAGS="-Wall -Werror" ./configure --with-asm=x86_64
   1<snip>
   2checking for x86_64 assembly availability... no
   3configure: error: x86_64 assembly requested but not available
   

   or

   0$ env CFLAGS="-Wall -Werror" cmake -B build -DSECP256K1_ASM=x86_64
   1<snip>
   2-- Performing Test HAVE_X86_64_ASM
   3-- Performing Test HAVE_X86_64_ASM - Failed
   4CMake Error at CMakeLists.txt:111 (message):
   5  x86_64 assembly requested but not available.
   6
   7
   8-- Configuring incomplete, errors occurred!
   

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

   This PR fixes both build systems.

2. hebasto renamed this:
   build: Fix `-Wuninitialized` warning in x86_64 assembly check code
   build: Fix `-Wuninitialized` warning in x86_64 assembly check
   on Sep 17, 2025
3. build: Fix warnings in x86_64 assembly check

   This change fixes:
   - `-Wuninitialized` in both Autotools and CMake;
   - `-Wreturn-type` in CMake only.

   8b75fa0807
4. hebasto force-pushed on Sep 17, 2025
5. hebasto renamed this:
   build: Fix `-Wuninitialized` warning in x86_64 assembly check
   build: Fix warnings in x86_64 assembly check
   on Sep 17, 2025

Contributors
hebasto