Use wildcard for libcxxabi file list

This will make it easier to update libcxxabi without requiring an
awkward cross-repo change when the list of source files changes.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3696
TEST=build libc++ (PNaCl toolchain trybots)

Review URL: https://codereview.chromium.org/77133005