guix: no-longer pass --enable-glibc-back-compat to Guix

Now that our Guix builds are performed on glibc 2.24 and 2.27 (RISCV),
we no-longer need to pass the --enable-glibc-back-compat option.

Replace it with --disable-threadlocal, to prevent the usage of symbols
from glibc 2.18.

None of the binaries produced required symbols later than 2.17, and 2.27
(RISCV).

Author: fanquake

contrib/guix/libexec/build.sh

@@ -240,7 +240,7 @@ mkdir -p "$OUTDIR"
 # CONFIGFLAGS
 CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests --disable-fuzz-binary"
 case "$HOST" in
-    *linux*) CONFIGFLAGS+=" --enable-glibc-back-compat" ;;
+    *linux*) CONFIGFLAGS+=" --disable-threadlocal" ;;
 esac
 
 # CFLAGS
@@ -260,6 +260,13 @@ case "$HOST" in
     *mingw*)  HOST_LDFLAGS="-Wl,--no-insert-timestamp" ;;
 esac
 
+# Using --no-tls-get-addr-optimize retains compatibility with glibc 2.17, by
+# avoiding a PowerPC64 optimisation available in glibc 2.22 and later.
+# https://sourceware.org/binutils/docs-2.35/ld/PowerPC64-ELF64.html
+case "$HOST" in
+    *powerpc64*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,--no-tls-get-addr-optimize" ;;
+esac
+
 case "$HOST" in
     powerpc64-linux-*|riscv64-linux-*) HOST_LDFLAGS="${HOST_LDFLAGS} -Wl,-z,noexecstack" ;;
 esac