[libunwind][ARM] Improve unwinder stack usage - Make WMMX support optional

These registers are only available on a limited set of ARM targets (those
based on XScale). Other targets should not have to pay the cost of these.

This patch shaves off about ~300 bytes of stack usage and ~1KB of code-size.

Differential revision: http://reviews.llvm.org/D21991
Reviewers: bcraig, compnerd

Change-Id: I2d7a1911a193bd70b123e78747e1a7d1482463c7
llvm-svn: 274744