databases/mongodb70: fix build on powerpc64le

In file included from src/third_party/abseil-cpp/dist/absl/base/internal/unscaledcycleclock.cc:27:
/usr/include/sys/sysctl.h:1189:25: error: unknown type name 'u_int'
int sysctl(const int *, u_int, void *, size_t *, const void *, size_t);

^

src/third_party/abseil-cpp/dist/absl/base/internal/unscaledcycleclock.cc:95:10: error: unknown type name 'once_flag'

static once_flag init_timebase_frequency_once;
       ^

src/third_party/abseil-cpp/dist/absl/base/internal/unscaledcycleclock.cc:97:18: error: no member named 'LowLevelCallOnce' in namespace 'absl::base_internal'

base_internal::LowLevelCallOnce(&init_timebase_frequency_once, [&]() {
~~~~~~~~~~~~~~~^

3 errors generated.