Add support of architecture specific ld.configs

Prefer architecture specific ld.config.<arch>.txt if it exists;
fall back to default ld.config.txt files if arch-specific one is
not present.

Bug: http://b/64061157
Test: bionic-unit-tests && linker-unit-tests
Change-Id: I83a298a932f2e4af7acb2049a7641fb86908736c