Build fails on ppc64le

Hi,
I've tried building this on a ppc64le machine (power8) and it fails in src/cycleclock.h:

In file included from /tmp/build/protobuf/third_party/benchmark/src/sysinfo.cc:44:0:
/tmp/build/protobuf/third_party/benchmark/src/cycleclock.h: In function 'int64_t benchmark::cycleclock::Now()':
/tmp/build/protobuf/third_party/benchmark/src/cycleclock.h:82:23: error: 'int64' does not name a type
tbl &= -static_cast(tbu0 == tbu1);

It looks like there is no 64bit ppc timebase instructions. The needed instruction is 'mfspr'. It returns a 64bit unsigned int and no need for low/high bit wrangling.

I'd submit a patch, but I haven't signed the CLA yet. Please let me know if you need a patch and I can get that started.