[Support] Adjust .note.GNU-stack guard in Support/BLAKE3/blake3_*_x86-64_unix.S (#76229)

When using GNU ld 2.41 on FreeBSD 14.0/amd64, there are linker warnings
like
```
/vol/gcc/bin/gld-2.41: warning: blake3_avx512_x86-64_unix.S.o: missing .note.GNU-stack section implies executable stack
/vol/gcc/bin/gld-2.41: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
```
This can be fixed by adjusting the guard of the `.note.GNU-stack`
sections in `blake3_*_x86-64_unix.S` to match `llvm/lib/MC/MCAsmInfoELF.cpp:MCAsmInfoELF::getNonexecutableStackSection` which emits the section on all ELF targets
but Solaris.

Tested on `amd64-pc-freebsd14.0`.