MIPS: Loongson64: test for -march=loongson3a cflag

Currently march flag is hardcoded to loongson3a for GCC and
mips64r2 for clang.

We can actually test it to simplify the code.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>