devel/pecl-grpc: fix build on powerpc64 elfv2

Clang doesn't have sys/platform/ppc.h:
/wrkdirs/usr/ports/devel/pecl-grpc/work-php74/grpc-1.27.0/third_party/abseil-cpp/absl/base/internal/unscaledcycleclock.cc:24:10: fatal error: 'sys/platform/ppc.h' file not found
#include <sys/platform/ppc.h>

^~~~~~~~~~~~~~~~~~~~

1 error generated.

ppc_get_timebase() and ppc_get_timebase_freq() are not supported on FreeBSD.

PR: 243995
Approved by: vanilla (maintainer)