[ARM] Create SubtargetFeatures from build attrs

An ELFObjectFile can now create SubtargetFeatures from the available
ARM build attributes, in a similar manner to MIPS. I've moved the
MIPS code into its own function and the ARM handler also has a
separate function.

Differential Revision: https://reviews.llvm.org/D28291



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292403 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed