[libc++abi] Create a macro for the 32 bit guard setting on ARM platforms

Summary:
The goal is to use a descriptive name for this feature, instead of just
using __arm__.

Reviewers: EricWF

Subscribers: javed.absar, kristof.beyls, christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60520

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@358106 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 1dc2c61c2a77bf864c7c627af72f1130a3e318ec)

Bug: http://b/189279320
Test: treehugger
Change-Id: I0360c4a2cd4f9f6e90dd176c4fa37fdfb817832b
3 files changed