gcc: remove "fixed" pthread.h

Fixes

    /nix/store/8pq96x5qsczrmc926795lf7j4dzy2c8q-binutils-patchelfed-ld-2.43.1/bin/ld: ../../src/liblzma/.libs/liblzma.so: undefined reference to `__pthread_cond_timedwait64'
    collect2: error: ld returned 1 exit status

while building `xz` for `stdenv`. The root cause is that glibc replaced
`__USE_TIME_BITS64` with `__USE_TIME64_REDIRECTS`[1]. However, the
stage3 GCC seems to use the bootstrap headers from glibc for
`fixincludes`, i.e. headers w/o this change. Because of that, the xz
build with stage3 stdenv gets a header that looks like this:

    # ifndef __USE_TIME_BITS64
    /* ... */
    # else
    #  ifdef __REDIRECT
    /* ... */
    #  else
    #   define pthread_cond_timedwait __pthread_cond_timedwait64
    #  endif
    # endif

Since __USE_TIME_BITS64 doesn't exist anymore because a new glibc is used
for building, the preprocessor ends up in the condition defining
`__pthread_cond_timedwait64` even though it's not supposed to end up
there since __pthread_cond_timedwait64 doesn't exist on x86_64[2].

I decided to just kill the header here since GCC claims that all the
`pthread.h` only very old glibc versions or platform-specific libcs we
don't use[3].

[1] https://sourceware.org/git/?p=glibc.git%3Ba%3Dcommit%3Bh%3Ddd535f4f19ef2b5c367a362af445ecadcf45401e
[2] https://inbox.sourceware.org/libc-stable/50c0269d-b73c-4e8a-9816-65f72d6082c0@linaro.org/
[3] https://github.com/gcc-mirror/gcc/blob/releases/gcc-14.2.0/fixincludes/inclhack.def

pkgs/development/compilers/gcc/common/builder.nix

@@ -303,7 +303,7 @@ originalAttrs: (stdenv.mkDerivation (finalAttrs: originalAttrs // {
     fi
 
     # Get rid of some "fixed" header files
-    rm -rfv $out/lib/gcc/*/*/include-fixed/{root,linux,sys/mount.h,bits/statx.h}
+    rm -rfv $out/lib/gcc/*/*/include-fixed/{root,linux,sys/mount.h,bits/statx.h,pthread.h}
 
     # Replace hard links for i686-pc-linux-gnu-gcc etc. with symlinks.
     for i in $out/bin/*-gcc*; do