x86-64 GCC CPU target options

[Cameron Gutman]

I noticed that GMP 5.1.2 doesn't use the newer GCC -march and -mtune
targets for Sandy Bridge (corei7-avx) and Haswell (core-avx2). Is this
because configure.ac just hasn't been updated or is there another reason?

Also, would it be worth splitting Ivy Bridge out of the coreisbr group so
we can use the core-avx-i GCC target?

I've tested a modified configure.ac that uses corei7-avx on an Ivy Bridge
system (recognized as coreisbr). It passes all the tests in make check. I
haven't done any major performance tests yet though.

Thanks,
Cameron