arm: semihosting: Fix returning from traps on ARMv6 and lower

U-Boot runs in supervisor mode. On ARMv6 and lower, software interrupts
are taken in supervisor mode. When entering an interrupt, the link
register is set to the address of the next instruction. However, if we
are already in supervisor mode, this clobbers the link register. The
debugger can't help us, since by the time it notices we've taken a
software interrupt, the link register is already gone. Work around this
by moving the return address to another register.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>