[ubsan] Support static linking with standalone runtime (#80943)

The standalone runtime (not
-fsanitize-minimal-runtime/-fsanitize-trap=undefined) installs some
signal handlers using `real_sigaction`. With static linking
(-static/-static-pie), the called `REAL(sigaction)` is null, leading to
an immediate segfault, which is confusing (#51538).

Fix #51538 by bailing out.

`// REQUIRES: librt_has_multf3` from https://reviews.llvm.org/D109709
actually disabled the test because `librt_has_*` features are only for
`compiler-rt/test/builtins`. The test does not reproduce for me:
libclang_rt.builtins.a or libgcc. Revert the REQUIRES.