tcg-arm: Improve GUEST_BASE qemu_ld/st

tcg-arm: Improve GUEST_BASE qemu_ld/st

If we pull the code to emit the actual load/store into a subroutine,
we can share the reg+reg addressing mode code between softmmu and
usermode.  This lets us load GUEST_BASE into a temporary register
rather than attempting to add it piece-wise to the address.

Which lets us use movw+movt for armv7, rather than (up to) 4 adds.
Code size for pre-armv7 stays the same.

Signed-off-by: Richard Henderson <rth@twiddle.net>