linux-user/i386: Use force_sig, force_sig_fault

Replace the local gen_signal with the generic functions that match
how the kernel raises signals. Fill in the missing PC for SIGTRAP.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220107213243.212806-11-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>