MIPS: Fallback CPU -march flag to ISA level if unsupported

LLVM does not implement some of -march options. However those options
are not mandatory for kernel to build for those CPUs.

Fallback -march CFLAG to ISA level if unsupported by toolchain so
we can get those kernel to build with LLVM.

Link: https://github.com/ClangBuiltLinux/linux/issues/1544


Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>