[ARM] The bits set in the variable HW_FP could get unset

when iterating through the Features vector if we don't
keep track of what's already been set. This could lead to
the macro __ARM_FP getting the wrong value. This patch
fixes this issue by keeping track of the bits that have
already been set in the loop.

Differential Revision: http://reviews.llvm.org/D10395

llvm-svn: 240607