[compiler-rt] [test] Allow expanding lit substitutions recursively

Summary:
This allows using lit substitutions in the `COMPILER_RT_EMULATOR` variable.

(For reference, the ability to expand substitutions recursively has been introduced in https://reviews.llvm.org/D76178.)

Reviewers: phosek, compnerd

Reviewed By: compnerd

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D83489