[nolibc] Unweak SymbolizerPrepareForSandboxing and move it to libc-independent part.

Fixes the Go build.

Differential Revision: http://llvm-reviews.chandlerc.com/D877

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182851 91177308-0d34-0410-b5e6-96231b3b80d8