New review comment by pullmoll on void-packages repository

https://github.com/void-linux/void-packages/pull/27412#discussion_r548874224

Comment:
267-275 could be simplified:
```
[ -z "$cross_libc" ] && msg_error "Undefined system libc (cross_libc)\n"
[ -z "$cross_${cross_libc}_version" ] && msg_error "Unknown $cross_libc version (cross_${cross_libc}_version)\n"
```

Perhaps then set *generic*
```
local cross_libc_version=${cross_${cross_libc}_version}
```