[compiler-rt] [test] Fix NameError when loading lit.cfg.py for crt

Summary:
The `execute_external` global variable is defined in [`lit.common.cfg.py`](https://github.com/llvm/llvm-project/blob/fcfb3170a776f89dde4de8ee105c99e10660f455/compiler-rt/test/lit.common.cfg.py#L18-L27) and used here (on lines 23 and 39). However, this variable is not visible in configs that are loaded independently.

Explicitly assign it to the correct value to avoid `NameError`.

Reviewers: compnerd, phosek

Reviewed By: compnerd, phosek

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79892