New issue by paper42 on void-packages repository

https://github.com/void-linux/void-packages/issues/37578

Description:
For example `libquadmath` is a subpackage of `gcc` that is only available for i686*, x86_64* and ppc64le, so `show-avail` should tell us it's not available for `aarch64-musl`.

### Expected behavior
```
%: ./xbps-src -a aarch64-musl show-avail libquadmath
%: echo $?
2
```

### Actual behavior
```
%: ./xbps-src -a aarch64-musl show-avail libquadmath
%: echo $?
0
```