[compiler-rt] Don't build ubsan cxxabi sources when unused

Summary:
On apple targets, when SANITIZER_CAN_USE_CXXABI is false,
the ubsan cxxabi sources aren't built, since they're unused.
Do this on non-apple targets as well.

This fixes errors when linking sanitizers if c++ abi is
unavailable.

Reviewers: pcc, kubabrecka, beanz

Subscribers: rnk, llvm-commits, kubabrecka, compnerd, dberris

Differential Revision: https://reviews.llvm.org/D23638

llvm-svn: 279467