Add MSan interceptor for fstat(2)

Summary:
Add new MSan interceptor that corrects NetBSD's
specific handling of fstat(2).

NetBSD renames the call to __fstat50.

Add new test: test/msan/fstat.cc

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D41637

llvm-svn: 321765