linux-user: Implement capability prctls

This is PR_CAPBSET_READ, PR_CAPBSET_DROP and the "legacy"
PR_CAP_AMBIENT PR_GET_SECUREBITS, PR_SET_SECUREBITS.

All of these arguments are integer values only, and do not
require mapping of values between host and guest.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220106225738.103012-5-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>