steps-guix: disable thread-dependent runtime pieces in gcc-15.2.0 pass1

The stage1 gcc build runs with --without-headers, but all-target-libgcc was
still pulling in gthr-default.h and failing on missing pthread.h.

Adjust steps-guix/gcc-15.2.0/pass1.sh configure flags to match a headers-free
bootstrap profile:
- add --disable-threads
- add --disable-shared
- add --disable-libssp
- add --disable-libsanitizer
- add --disable-libquadmath
- add --disable-libatomic
- add --disable-libgomp

Keep the existing multilib bootstrap flow:
- make all-gcc
- make all-target-libgcc
- make install-gcc
- make install-target-libgcc

steps-guix/gcc-15.2.0/pass1.sh

@@ -265,6 +265,13 @@ src_configure() {
         --disable-nls \
         --enable-languages=c \
         --without-headers \
+        --disable-threads \
+        --disable-shared \
+        --disable-libssp \
+        --disable-libsanitizer \
+        --disable-libquadmath \
+        --disable-libatomic \
+        --disable-libgomp \
         --enable-multilib
 }