Auto merge of #31810 - gandro:netbsd-fix-stat, r=alexcrichton

Some struct members have a slighty different name on NetBSD. This has been fixed in the libc crate, but not in libstd, breaking the NetBSD build. Related C struct definition: http://nxr.netbsd.org/xref/src/sys/sys/stat.h?r=1.68#59

This also removes the broken `st_spare()` from MetadataExt, since it is private field reserved for future use.

@dhuseby In case this conflicts with any of your pending patches, feel free to intervene - I'm happy to withdraw this PR.

r? @alexcrichton