New review comment by ifreund on void-packages repository

https://github.com/void-linux/void-packages/pull/29288#discussion_r747375794

Comment:
according to the output of `zig libc`, `gcc_dir` is only needed when targeting haiku:
```
# The directory that contains `crtbeginS.o` and `crtendS.o`
# Only needed when targeting Haiku.
gcc_dir=
```