Temporary add ld.config.txt build

Last generic ld.config.txt deprecation caused build error from
build_test build because of invalid ld.config.txt build target. To fix
this ld.config.txt build target should be enabled until
ld.config.vndklite.txt and ld.config.legacy.txt also become deprecated.

Bug: 138920271
Test: m ld.config.txt passed
Test: build_test passed
Change-Id: I7f45ee66d3ed70a1a698261e1e801a30729f592f