[nolibc] Begin moving sanitizer_common's libc-dependent code to a separate library

Introduce a new object library, RTSanitizerCommonLibc, which will contain
the subset of sanitizer_common with libc dependencies. RTSanitizerCommon
contains the remainder of sanitizer_common, and is intended to have no
libc dependencies. Begin moving code to RTSanitizerCommonLibc, starting
with sanitizer_common.cc, whose libc-dependent portion is moved to
sanitizer_common_libcdep.cc, the first member of the new library.

This split affects the CMake build only.  The makefile build continues
to produce the full sanitizer_common library.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182118 91177308-0d34-0410-b5e6-96231b3b80d8
[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.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182132 91177308-0d34-0410-b5e6-96231b3b80d8