make gpr_timespec platform agnostic

include/grpc/support/time.h

src/core/iomgr/timer.c

src/core/profiling/basic_timers.c

src/core/security/credentials.c

src/core/statistics/window_stats.c

src/core/support/log_linux.c

src/core/support/log_posix.c

src/core/support/log_win32.c

src/core/support/time.c

src/core/support/time_posix.c

src/core/support/time_precise.c

src/core/support/time_win32.c

src/core/surface/channel.c

src/core/surface/channel_connectivity.c

src/core/surface/completion_queue.c

src/core/transport/chttp2/timeout_encoding.c

src/core/transport/transport_op_string.c

src/cpp/util/time.cc

src/csharp/Grpc.Core.Tests/Internal/TimespecTest.cs

src/csharp/Grpc.Core/Internal/Timespec.cs