tools headers uapi: Sync copy of asm-generic/unistd.h with the kernel sources

To get the changes in:

  c8ce48f0 ("asm-generic: Make time32 syscall numbers optional")

Silencing these tools/perf build warnings:

  Warning: Kernel ABI header at 'tools/arch/arm64/include/uapi/asm/unistd.h' differs from latest version at 'arch/arm64/include/uapi/asm/unistd.h'
  diff -u tools/arch/arm64/include/uapi/asm/unistd.h arch/arm64/include/uapi/asm/unistd.h
  Warning: Kernel ABI header at 'tools/include/uapi/asm-generic/unistd.h' differs from latest version at 'include/uapi/asm-generic/unistd.h'
  diff -u tools/include/uapi/asm-generic/unistd.h include/uapi/asm-generic/unistd.h

Test built it under the ubuntu:14.04.4-x-linaro-arm64 cross build
environment and looked at the syscall table at
/tmp/build/perf/arch/arm64/include/generated/asm/syscalls.c, looks ok.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Hendrik Brueck...