[RuntimeDyld][MachO] Properly handle thumb to thumb calls within a section.

Previously thumb bits were only checked for external relocations (thumb to arm
code and vice-versa). This patch adds detection for thumb callees in the same
section asthe (also thumb) caller.

The MachO/Thumb test case is updated to cover this, and redundant checks
(handled by the MachO/ARM test) are removed.

llvm-svn: 331838