Get APExp to build on different architectures and non-9front Plan9 distributions

I build and verify that the source tree builds from scratch on a clean 9front amd64 VM before each release. The purpose of APExp is to be able to build on as many architectures as possible, and ideally also on different Plan9 distributions. Being portable across distributions ensures that we have included enough in APExp so that the project is less sensitive to changes in the underlying host os.

APExp has been verified to build (and work) from scratch on:

| Builds | OS | Architecture | Verified to build/cross-compile since release | Able to self-host? |
| ------------- | ------------- |------------- |------------- |------------- |
| [ ] | 9front | 386 | N/A | ? |
| [ ] | 9front | 68k | N/A | ? |
| [x] | 9front | amd64 | 0.1 | yes |
| [ ] | 9front | arm | N/A | ? |
| [ ] | 9front | arm64 | N/A | ? |
| [ ] | 9front | mips | N/A | ? |
| [ ] | 9front | power | N/A | ? |
| [ ] | 9front | power64 | N/A | ? |
| [ ] | 9front | sparc | N/A | ? |
| [ ] | 9front | sparc64 | N/A | ? |
| [ ] | 9front | spim | N/A | ? |
| [ ] | 9legacy | 386 | N/A | ? |
| [ ] | 9legacy | amd64 | N/A | ? |
| [ ] | 9legacy | arm | N/A | ? |