libc/tests: Fix installation without MK_TOOLCHAIN

There is some exotic conditional logic here to avoid building a
particular test if a certain UBSAN library isn't present in the
toolchain sysroot. This causes build failures for me when doing an
"installworld WITHOUT_TOOLCHAIN=", which I do frequently during tests.

I believe the problem is that SYSROOT is unset during installworld, so
the build sees the host's copy of libclang_rt.ubsan_standalone.a and
then tries to install a binary that wasn't built during buildworld. Try
to make the check a bit less fragile.

Reviewed by: dim
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D45035