libsanitizer: Use pre-computed size of struct ustat for Linux

libsanitizer: Use pre-computed size of struct ustat for Linux

Cherry-pick compiler-rt revision 333213:

<sys/ustat.h> has been removed from glibc 2.28 by:

commit cf2478d53ad7071e84c724a986b56fe17f4f4ca7
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Sun Mar 18 11:28:59 2018 +0800

    Deprecate ustat syscall interface

This patch uses pre-computed size of struct ustat for Linux.

	PR sanitizer/85835
	* sanitizer_common/sanitizer_platform_limits_posix.cc: Don't
	include <sys/ustat.h> for Linux.
	(SIZEOF_STRUCT_USTAT): New.
	(struct_ustat_sz): Use SIZEOF_STRUCT_USTAT for Linux.

From-SVN: r260684