[MC][ARM] Correct Thumb BL instruction range

The Thumb BL range is + or - either 16 Megabytes or 4 Megabytes depending
on whether the CPU supports Thumb2 or the v8-m baseline ops. The existing
check for BL range is incorrectly set at +- 32 Megabytes. This change
corrects the higher range and uses the lower range if the featurebits
don't have the necessary support for it.

Differential Revision: https://reviews.llvm.org/D46305

llvm-svn: 333991