linux-user/riscv: Adjust vdso signal frame cfa offsets

linux-user/riscv: Adjust vdso signal frame cfa offsets

A typo in sizeof_reg put the registers at the wrong offset.

Simplify the expressions to use positive addresses from the
start of uc_mcontext instead of negative addresses from the
end of uc_mcontext.

Reported-by: Vineet Gupta <vineetg@rivosinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>