Add clang-real to makevars.go

clang-real will replace clang.real so we should copy it over for ART
tests. This was the reason for ART tests failing in b/432338117 when we
created new prebuilts. `clang.real` will be removed in a subsequent
change.

Flag: EXEMPT bugfix
Bug: b/269697259
Test: presubmit and ab/P100712307
Change-Id: I22e966653cafd80f2d045277da6d4f69a53b2813