[nolibc] Move all platforms to internal_getpid.

Before, we had an unused internal_getpid function for Linux, and a
platform-independent GetPid function.  To make the naming conventions
consistent for syscall-like functions, the GetPid syscall wrapper
in sanitizer_posix.cc is moved to sanitizer_mac.cc, and GetPid is
renamed to internal_getpid, bringing the Linux variant into use.