sanitizer_common: fix buildbot by restoring wcslen semantics

Windows buildbot (https://lab.llvm.org/buildbot/#/builders/127/builds/48479)
was broken by my patch https://reviews.llvm.org/D150909 because it
gated wcslen interception to SI_POSIX, instead of always intercepting wcslen.
This changes it back to always intercepting wcslen.