[sanitizer_common] Don't provide sanitizer_procmaps API functions where not defined

Summary:
Platforms that don't implement procmaps (primarily fuchsia and windows) still expose
the procmaps API when including sanitizer_procmaps.h, despite not implementing the functions
provided by that header. Ensure that the API is only exposed on platforms that implement it.

Reviewers: vitalybuka, alekseyshl, kubamracek

Subscribers: llvm-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D38187

llvm-svn: 314149