[ARM] Add support for ARMV6K subtarget (LLVM)

ARMv6K is another layer between ARMV6 and ARMV6T2. This is the LLVM
side of the changes.

ARMV6 family LLVM implementation.

+-------------------------------------+
| ARMV6                               |
+----------------+--------------------+
| ARMV6M (thumb) | ARMV6K (arm,thumb) | <- From ARMV6K and ARMV6M processors
+----------------+--------------------+    have support for hint instructions
| ARMV6T2 (arm,thumb,thumb2)          |    (SEV/WFE/WFI/NOP/YIELD). They can
+-------------------------------------+    be either real or default to NOP.
| ARMV7 (arm,thumb,thumb2)            |    The two processors also use
+-------------------------------------+    different encoding for them.

Patch by Vinicius Tinti.

llvm-svn: 232468