[ARM] Remove non-existent cpu arm1176j-s and use mpcore for v6k

The ARMTargetParser.def contains an entry for arm1176j-s which is the
default for the ArmV6K architecture. This cpu does not exist, there are
only arm1176jz-s and arm1176jzf-s and they are both architecture ArmV6KZ.
The only CPUs that are actually ArmV6K are the mpcore, mpcore_nofpu and
later revisions of the arm1136 family r1px (which we don't have a table
entry for).

This patch removes the arm1176j-s and makes mpcore the default for armv6k.

Differential Revision: https://reviews.llvm.org/D52594

llvm-svn: 343303