Register the local 32-bit C++ toolchain with highest priority.

Change-Id: I79d52e24ed00e875d1cfbb1f316139ad7639aff6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62470
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>