[compiler-rt] Avoid pulling in __cxa_pure_virtual

When building optimized versions of the runtime libraries the compiler
is generally able to elide these references, but when building them
for maximum debug info (with -O0), these references remain which causes
the test suite to fail for tests that do not pull in the C++ standard
library.

Reviewed By: vitalybuka

Pull Request: https://github.com/llvm/llvm-project/pull/84613