Solarish: Implement file system support

[RalfJung]

Our Solarish support is overall pretty good, but one main thing that is missing is filesystem support. Solarish is probably very close to the other Unixes we support here, so this shouldn't be very hard.

The test command for this is: ./miri test --target x86_64-unknown-illumos fs.

[devnexen]

Note to myself: To complete this and finally enable the rest of the fs tests, still need to support readdir(64) and maybe readdir_r (not sure about this one).