LoongArch: Use "$fcsr0" instead of "$r0" in _FPU_{GET,SET}CW

Clang inline-asm parser does not allow using "$r0" in
movfcsr2gr/movgr2fcsr, so everything using _FPU_{GET,SET}CW is now
failing to build with Clang on LoongArch.  As we now requires Binutils
>= 2.41 which supports using "$fcsr0" here, use it instead of "$r0" to
fix the issue.

Link: https://github.com/loongson-community/discussions/issues/53#issuecomment-2081507390
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=4142b2368353


Signed-off-by: Xi Ruoyao <xry111@xry111.site>