[llvm-exegesis] Change how the native architecture is determined

[llvm-exegesis] Change how the native architecture is determined

Currently the llvm-exegesis native architecture is determined by comparing the
llvm native architecture with X86, so to add a new target would mean adding a
new check. Change this to building up a list of the targets llvm-exegesis
supports then using that, as this means that when adding a new target you just
add the target to the list of supported targets.

Differential Revision: https://reviews.llvm.org/D48778

llvm-svn: 336105