[libFuzzer] Disable MSan interceptors in SIGINT handler.

Summary:
Avoids an MSan false positive if the SIGINT comes while the user
callback is running.  The false positive happens when the interrupt
handler calls opendir() to remove some temporary files, which is
intercepted by MSan.

Fixes https://github.com/google/oss-fuzz/issues/2332.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits, Dor1s, metzman

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61163

llvm-svn: 359254