arm: semihosting: Support semihosting fallback on 32-bit ARM

Add support for a semihosting fallback on 32-bit ARM. The assembly is
lightly adapted from the irq return code, except there is no offset
since lr already points to the correct instruction. The C side is mostly
like ARM64, except we have fewer cases to deal with.

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