[ARM] Add ACLE feature macros for MVE

If MVE is present at all, then the macro __ARM_FEATURE_MVE is defined
to a value which has bit 0 set for integer MVE, and bit 1 set for
floating-point MVE.

(Floating-point MVE implies integer MVE, so if this macro is defined
at all then it will be set to 1 or 3, never 2.)

Patch mostly by Simon Tatham

Differential Revision: https://reviews.llvm.org/D60710

llvm-svn: 362806