Fix llvm mac build

[hughperkins]

see failures such as https://github.com/Genesis-Embodied-AI/gstaichi/actions/runs/18300984728/job/52108893427?pr=216

binary search on the llvm builds found:

202509261351 => fail
202509261125 => fail
202509212040 => ok

202509202046 => ok

Using this branch:

202510070835 (remove strip) => ok
202510070832 (baseline: no change) => fail

This PR removes strip from mac build, which fixes the gstaichi mac build.

Size of mac os x llvm asset chagnes from 696MB to 784MB, which seems an acceptable-ish change, for a build that works.

Note that the artifacts from this build are not directly user-visible for gstaichi users:

• this build will be linked with the c++ code from gstaichi
• then we run strip the resulting wheel

[hughperkins]

Thanks!