tools arch x86 uapi: Synch asm/unistd.h with the kernel sources

To pick up the change in:

  700d3a5a ("x86/syscalls: Revert "x86/syscalls: Make __X32_SYSCALL_BIT be unsigned long"")

That doesn't trigger any changes in tooling and silences this perf build
warning:

  Warning: Kernel ABI header at 'tools/arch/x86/include/uapi/asm/unistd.h' differs from latest version at 'arch/x86/include/uapi/asm/unistd.h'
  diff -u tools/arch/x86/include/uapi/asm/unistd.h arch/x86/include/uapi/asm/unistd.h

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/arch/x86/include/uapi/asm/unistd.h

@@ -2,7 +2,14 @@
 #ifndef _UAPI_ASM_X86_UNISTD_H
 #define _UAPI_ASM_X86_UNISTD_H
 
-/* x32 syscall flag bit */
+/*
+ * x32 syscall flag bit.  Some user programs expect syscall NR macros
+ * and __X32_SYSCALL_BIT to have type int, even though syscall numbers
+ * are, for practical purposes, unsigned long.
+ *
+ * Fortunately, expressions like (nr & ~__X32_SYSCALL_BIT) do the right
+ * thing regardless.
+ */
 #define __X32_SYSCALL_BIT	0x40000000
 
 #ifndef __KERNEL__