Disable recursive interceptors in signal(3)/MSan

Summary:
signal(3) on NetBSD calls internally sigaction(2).

Without disabling the recursive interceptor, there are
false positives about uninitialized memory reads inside libc.

This change fixes execution of such programs as sh(1) and
vmstat(1) in the NetBSD userland.

Sponsored by <The NetBSD Foundation>

Reviewers: eugenis, vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D47650

llvm-svn: 334168