[safestack] Remove dependency of SafeStack on sanitizer_common

Summary:
SafeStack needs just few functions from there, but sanitizer_common
introduces conflicts with other runtimes, e.g. SCUDO.

Reviewers: eugenis, kcc, cryptoad

Subscribers: mgorny, krytarowski, fedor.sergeev, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D56886

llvm-svn: 351506