[compiler-rt] Replace allow_user_segv_handler=0 with kHandleSignalExclusive

Summary:
allow_user_segv_handler had confusing name did not allow to control behavior for
signals separately.

Reviewers: eugenis, alekseyshl, kcc

Subscribers: llvm-commits, dberris, kubamracek

Differential Revision: https://reviews.llvm.org/D33371

llvm-svn: 303941