MIPS: gas: alter 64 or 32 for mipsisa triples if march is implicit

When configure with triples mipsisa[32,64]rN[el,], the march value
is pinned to a fix value if not given explicitly. for example
   1) mipsisa32r6-linux-gnu -n32 xx.s will complains that:
      -march=mips32r6 is not compatible with the selected ABI
   2) mipsisa64r2el-linux-gnu -o32 generates objects with 64bit CPU:
      ELF 32-bit LSB relocatable, MIPS, MIPS64 rel2 version 1 (SYSV)
They are not good default behaviors: Let's alter the CPU info

Since we are using these triples as a regular linux distributions,
let's alter march according to ABI.