Provide support for armv4 and armv5, and fix assembler warnings

Provide untested support for armv4 and armv5 (selected by -DARMV4
or -DARMV5 in the Makefile) by mapping the "bx lr" instruction
to "mov pc, lr" on platforms earlier than armv6.

Fix assembler warnings for conditional infixes in copy_from/to_user.