[libc++] Workaround timespec_get not always being available in Apple SDKs

timespec_get is not available in Apple SDKs when (__DARWIN_C_LEVEL >= __DARWIN_C_FULL)
isn't true, which leads to libc++ trying to import ::timespec_get into
namespace std when it's not available. This issue has been reported to
Apple's libc, but we need a workaround in the meantime.

https://llvm.org/PR47208
rdar://68157284