common-user: Move safe-syscall.* from linux-user

Move linux-user safe-syscall.S and safe-syscall-error.c to common-user
so that bsd-user can also use it.  Also move safe-syscall.h to
include/user/.  Since there is nothing here that is related to the guest,
as opposed to the host, build it once.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

bsd-user/meson.build

@@ -2,6 +2,8 @@ if not have_bsd_user
    subdir_done()
 endif
 
+common_user_inc += include_directories('.')
+
 bsd_user_ss.add(files(
   'bsdload.c',
   'elfload.c',