Add realpath implementation to sysbase

[oreo639]

This adds a realpath() implementation to sysbase. Currently, the function for resolving the path is basically the same as used for chdir() (but without the / automatically added to the end ofc), if that needs to be changed, let me know.

One issue with this PR rn that it duplicates macros/variables defined in chmod.c, which I assume is not desirable. I'm not sure how best to fix that. Any suggestions would be appreciated.

If there are any other issues, please let me know.

[WinterMute]

I ended up going a bit further with the realpath implementation and allowing for symlinks here https://github.com/devkitPro/newlib/blob/devkitPro/libgloss/libsysbase/realpath.c

I did start from your implementation though so thanks for the PR

[oreo639]

Thanks.