build: add linker optimization flags to guix

Any -O argument will enable optimizations in GNU ld. We can use -O2
here, as this matches our compile flags. Note that this would also
enable additional optimizations if using the lld or gold linkers,
when compared to -O0.

contrib/guix/libexec/build.sh

@@ -177,7 +177,7 @@ HOST_CXXFLAGS="$HOST_CFLAGS"
 
 # LDFLAGS
 case "$HOST" in
-    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -static-libstdc++" ;;
+    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -static-libstdc++ -Wl,-O2" ;;
     *mingw*)  HOST_LDFLAGS="-Wl,--no-insert-timestamp" ;;
 esac