Revert^2 "Remove external/libcxx in favor of prebuilt"

This CL reverts commit 45e6c488a5ad73455d2b70c2ef7d9783cf893dfd.

Bug: 332594828
Test: treehugger
Change-Id: Ifcc2d0149d9765dac0b539f249dbf3cc80fe110b
1 file changed