Revert "Remove external/libcxx in favor of prebuilt"

Revert submission 2675705-use-prebuilt-libcxx

Reason for revert: breaks some Android tests (b/337120479, b/337121737, b/337122511)

Reverted changes: /q/submissionid:2675705-use-prebuilt-libcxx

Bug: 337120479
Bug: 337121737
Bug: 337122511
Change-Id: Ie06cb6c61a08c5f2119231ac8eaad8f8069e620a
1 file changed