[ARM] Preserve fpu behaviour for '-crypto'

Summary:
This patch restores the behaviour that -fpu overwrites the
architecture obtained from -march or -mcpu flags, not enforcing to
disable 'crypto' if march=armv7 and mfpu=neon-fp-armv8.
However, it does warn that 'crypto' is ignored when passing
mfpu=crypto-neon-fp-armv8.

Reviewers: peter.smith, labrinea

Reviewed By: peter.smith

Subscribers: nickdesaulniers, kristof.beyls, dmgreen, cfe-commits, krisb

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67608

llvm-svn: 374785