[ASan] Introduce SymbolizerPrepareForSandboxing(), which is a no-op on every platform except Linux (because we don't support sandboxing anywhere else yet)
On Linux we pre-cache the value of readlink("/proc/self/exe"), so that it can be later used when the sandbox has been turned on.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182579 91177308-0d34-0410-b5e6-96231b3b80d8