[PATCH] MIPS: gas: alter 64 or 32 for r6 triples if march is implicit

[YunQiang Su]

Let's be back to the problem of this patch: what should we do for
     mipsisa32rXel-*-as -64
and
    mipsisa64rXel-*-as -32
We have several options:
1. Keep current, aka issue an error message and exit for 32-64 combination.
2. set the default arch to mips64r6 only for mipsisa32r6, aka not for
mipsisa32r[1-5].
    set the default arch to mips32r6 only for mipsisa64r6, aka not for
mipsisa32r[1-5].
    That's what I did in this patch.
3. set the default arch to mips64rN for mipsisa32rN
    set the default arch to mips32rN for mipsisa64rN
    but do nothing for any other triples, such as
       mipstx39* or mips64vr,
    Since it's hard to determine which arches are best for them.
4. Set the default 32/64 arch for all triples.