[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

llvm-svn: 358106