RISC-V: reduce mcount save space on RV32

For RV32 we can reduce the size of the ABI save+restore state by using
SZREG so that register stores are packed rather than on an 8 byte
boundary.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20221115200832.706370-3-jamie@jamieiles.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>