tests/tcg/i386: Introduce and use reg_t consistently

tests/tcg/i386: Introduce and use reg_t consistently

Define reg_t based on the actual register width.
Define the inlines using that type.  This will allow
input registers to 32-bit insns to be set to 64-bit
values on x86-64, which allows testing various edge cases.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230114230542.3116013-2-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>