Inline ldrex/strex for thumb2, remove ARMv5 remnants.

We assume ARMv7 which with thumb2 has ldrex/strex operations that may be
inlined if -mthumb is defined.
Remove remnants of ARMv5 support.

(cherry picked from commit 80c3b278)

Change-Id: I194a3d08e933c63f79d48288d9235b4e64099340