Create a make variable for all lsdump files.

This is needed so that we can embed the paths of lsdump files in a file,
in order to make lookup faster, while creating reference dumps.

Test: mm -j64 in external/libjpeg-turbo for aosp_arm64_ab;
      out/soong/make_vars-aosp_arm64_ab.mk contains
      SOONG_LSDUMP_PATHS := <list of lsdump paths>

Change-Id: I04608429e1add307cc1ee79d2f0c348fb041613c