linux-user: Add host_signal_set_pc to set pc in mcontext

linux-user: Add host_signal_set_pc to set pc in mcontext

Add a new function host_signal_set_pc to set the next pc in an
mcontext. The caller should ensure this is a valid PC for execution.

Acked-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211113045603.60391-2-imp@bsdimp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>