darwin: Silence syscall deprecated declaration warning

darwin: Silence syscall deprecated declaration warning

This patch silences this warning:

/Users/simark/src/binutils-gdb/gdb/darwin-nat.c:839:10: error: 'syscall' is deprecated: first deprecated in macOS 10.12 - syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use kdebug_signpost(). [-Werror,-Wdeprecated-declarations]
          res = syscall (SYS___pthread_kill, thread->gdb_port, nsignal);
                ^
/usr/include/unistd.h:745:6: note: 'syscall' has been explicitly marked deprecated here
int      syscall(int, ...);
         ^

The comment of the new pthread_kill function explains why we use the
syscall function directly.

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_DECLARATIONS):
	Define for clang.

gdb/ChangeLog:

	* darwin-nat.c (darwin_pthread_kill): New function.
	(darwin_resume_thread): Use darwin_pthread_kill.