[AARCH64] Add scheduler for ThunderX

This adds the schedule model for ThunderX. There are a few TODOs in that
not all of the SIMD is model currently.  Also the idea of a simple
shift/extend is not modeled and all cases where there is a shift/extend
is considered as non simple and take up two cycles rather than correct
value of one cycle.  Also the 32bit divide and the 64bit divide
have different cycle counts but there is no way to model that currently.
Also multiply high takes one cycle more than the normal multiply but
there is no way to model that currently either.

Build and tested for aarch64-elf with no regressions.

ChangeLog:
* config/aarch64/aarch64-cores.def (thunderx): Change the scheduler
over to thunderx.
* config/aarch64/aarch64.md: Include thunderx.md.
(generic_sched): Set to no for thunderx.
* config/aarch64/thunderx.md: New file.

From-SVN: r217593