Fix copying lsdumps when TARGET_ARCH == TARGET_ARCH_VARIANT.

Bug: 64267858

Test: utils/create_reference_dumps.py -l libc++ for aosp_x86_64_ab (here
      TARGET_ARCH == TARGET_ARCH_VARIANT), copies libc++.so.lsdump to
      prebuilts/abi-dumps/vndk/current/x86_64/x864_64/source-based/libc++.so.lsdump

Merged-In: I8ec48bcfa5cf1487ac1f6e904c1b4cea70719aa3
Change-Id: I8ec48bcfa5cf1487ac1f6e904c1b4cea70719aa3
(cherry picked from commit de24e5dd)