[asan] Include <sys/stat.h> if __x86_64__ is defined

struct stat defined in <asm/stat.h> is incorrect for x32.  <asm/stat.h>
is included to get struct __old_kernel_stat.  But struct __old_kernel_stat
isn't used for x86-64 nor x32.  This patch includes <sys/stat.h> instead
of <asm/stat.h> and comments out size check of struct __old_kernel_stat
for x86-64.

Patch by H.J. Lu