Some targets (e.g. Alpha and MIPS64) need to keep 32-bit operands
sign-extended in 64-bit registers (regardless of the "real" sign
of the operand).  For that, we need to be able to distinguish
between a 32-bit load with a 32-bit result and a 32-bit load with
a given extension to a 64-bit result.  This distinction already
exists for the ld* loads, but not the qemu_ld* loads.

Reserve qemu_ld32u for 64-bit outputs and introduce qemu_ld32 for
32-bit outputs.  Adjust all code generators to match.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>