RISC-V: preserve a1 in mcount

RISC-V: preserve a1 in mcount

The RISC-V ELF psABI states that both a0 and a1 are used for return
values so we should preserve them both in return_to_handler.  This is
especially important for RV32 for functions returning a 64-bit quantity
otherwise the return value can be corrupted and undefined behaviour
results.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Link: https://lore.kernel.org/r/20221115200832.706370-4-jamie@jamieiles.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>