Hello,

This patch fixes libgccjit link failure on loongarch* targets,
and could probably be useful for future ports.

Currently libgccjit is linked with objects from $(EXTRA_GCC_OBJS) and
libbackend.a, which is in turn linked with $(EXTRA_OBJS) files.
Thus, common object files that's shared between those two $(EXTRA_*)
lists would cause the linker to abort for "redefined symbol"s.

For now, this patch doesn't affect any target other than LoongArch,
but might be useful for future ports that want to share more object
files between the compiler proper and the GCC driver than
${cpu_arch}-common.o.

Regression tested on loongarch64-linux-gnuf64 an x86_64-pc-linux-gnu, 
please review.

Yujie