Fix __ARM_FP value for sp-only FPUs with Half-precision

The logic for parsing FP capabilities to set __ARM_FP was mistakenly removing
the Half-Precision capability when handling fp-only-sp resulting in a value
of 0x4. Section 6.5.1 of ACLE states that for such FP architectures the value
should be 0x6

llvm-svn: 250888