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.