Bug 1426283 - Use nproc for the number of parallel jobs to run when building gcc. r=gps

While in the vicinity.

--HG--
extra : rebase_source : 8d885ff5fd4fa124a08326c0ca002a49ef559e57

build/unix/build-gcc/build-gcc.sh

@@ -3,7 +3,7 @@
 set -e
 set -x
 
-make_flags='-j12'
+make_flags="-j$(nproc)"
 
 . $data_dir/download-tools.sh