[sanitizer_common] Disable filesystem-related code for Fuchsia

Summary:
Fuchsia doesn't support filesystem access per se at low level.
So it won't use any of the filesystem-oriented code in sanitizer_common.

Submitted on behalf of Roland McGrath.

Reviewers: vitalybuka, alekseyshl, kcc

Reviewed By: vitalybuka

Subscribers: kubamracek, phosek, filcab, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36029

llvm-svn: 309749