[AArch64] Enable aggressive FMA on T99 and provide AArch64 options for others.

This patch enables aggressive FMA by default on T99, and provides a -mllvm
option to enable the same on other AArch64 micro-arch's (-mllvm
-aarch64-enable-aggressive-fma).

Test case demonstrating the effects on T99 is included.

Patch by: steleman (Stefan Teleman)

Differential Revision: https://reviews.llvm.org/D40696

llvm-svn: 323474