Properly generate lists of exported symbols for sanitizer runtimes

This change adds a Python script that is invoked for
the just-built sanitizer runtime to generate the list of exported symbols
passed to the linker. By default, it contains interceptors and sanitizer
interface functions, but can be extended with tool-specific lists.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@189356 91177308-0d34-0410-b5e6-96231b3b80d8