[compiler-rt] Don't force ASAN_HAS_EXCEPTIONS to be true for all builds

Summary:
This value is already defaulted to true in asan_internal.h.
Allow the value to be overriden in cases where exceptions are unavailable.

Reviewers: kcc, samsonov, compnerd

Subscribers: kubabrecka, dberris, beanz, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D24633

llvm-svn: 281746