MIPS: Makefile: Pass -march option on Loongson3A cores

The loongson 3A cores do not select a suitable -march option so the build
system uses the default one from the toolchain. This may or may not be
suitable for a loongson 3A build. In order to avoid that, we explicitly set
a suitable -march option for that core.  Furthermore, some very old
compilers don't support -march= at all and there is the possibility of
toolchain combinations such as GCC 4.9 and binutils 2.24 for which
-march=loongson3a will result in MIPS64 R2 code being generated but then
rejected by GAS.  So treat the Longsoon 3A as an R2 CPU.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>