Merge Linux and FreeBSD arm register contexts

Summary:
These two register contexts were identical, so this shouldn't cause any
regressions, but I'd appreciate it if you can check that this at least compiles.

Reviewers: emaste, sas

Subscribers: aemerson, rengolin, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D27126

llvm-svn: 296335