[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.