cleanup: Remove hardcoded `compiler-builtins-mem` feature for `armv7a-vex-v5` once `compiler-builtins` can automatically add it.

[tropicaaal]

PR #131530 adds the armv7a-vex-v5 Tier 3 target, which operates outside of a libc enviornment. As such as we aren't provided symbols like memcpy, memmove, etc... and require the compiler-builtins-mem feature. In the PR, we hardcode bootstrap to automatically add the feature when target.contains("vex"). The zkvm family of targets also does this.

Once compiler-builtins is able to automatically detect the target, this check should be removed.

[tropicaaal]

Closing due to #131530 being closed and hopefully split into smaller changes in the future. This will be addressed at a later time.