gcc: enable libgomp for musl targets

libgomp has been disabled on musl since musl support was first added to
nixpkgs (15d401dcfa), but seems to work
fine. Tested down to gcc 6 (gcc 4.8 already doesn't build for musl)

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -93,6 +93,7 @@ let
       # libsanitizer requires netrom/netrom.h which is not
       # available in uclibc.
       "--disable-libsanitizer"
+    ] ++ lib.optionals (targetPlatform.libc == "uclibc") [
       # In uclibc cases, libgomp needs an additional '-ldl'
       # and as I don't know how to pass it, I disable libgomp.
       "--disable-libgomp"