[asan] Align __asan_global_start so that it works with LLD

Otherwise LLD will not align the .ASAN$GA section start, and
&__asan_globals + 1 will not be the start of the next real ASan global
metadata in .ASAN$GL.

We discovered this issue when attempting to use LLD on Windows in
Chromium: https://crbug.com/837090

llvm-svn: 330990