Fix ARMv4 support

ARMv4 doesn't support the "BX" instruction, which has been introduced
with ARMv4t. Adjust the call lowering and tail call implementation
accordingly.

Further changes are necessary to ensure that presence of the v4t feature
is correctly set. Most importantly, the "generic" CPU for thumb-*
triples should include ARMv4t, since thumb mode without thumb support
would naturally be pointless.

Add a couple of asserts to ensure thumb instructions are not emitted
without CPU support.

Differential Revision: https://reviews.llvm.org/D37030

llvm-svn: 311921