[symbolizer] Change libcxx paths in buildscript.

D107799 changed the paths from lib/libcxx(abi)?.a to
lib/<triple>/libcxx(abi)?.a. The build script needs to know to pick up
the files from the triple subfolder instead.

See https://lab.llvm.org/buildbot/#/builders/37/builds/6764 for buildbot log failure.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D109924