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>
diff --git a/MAINTAINERS b/MAINTAINERS
index 9a8d1bd..be8fc57 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3079,6 +3079,8 @@
 S: Maintained
 F: thunk.c
 F: accel/tcg/user-exec*.c
+F: include/user/
+F: common-user/
 
 BSD user
 M: Warner Losh <imp@bsdimp.com>