Fix illumos build regressions

[am11]

• gcc looks for fallthrough or break in morph (we could move the break out of condition, but there are a few gotos to that case which I am not sure might have sideeffect, so i kept it simple).
• SHM_ANON is not available in illumos/Solaris libc (POSIX does not require it), so manually generate name using timespec for shm_open() call.
• in line with previous fixes made for statfs, SunOS-like OS has deprecated statfs in favor of statvfs. The f_type does not return magic number as other Unices. The more reliable way is to get the stringly name from f_basetype and then resolve the enum value. Assuming performance is not a concern for DriveInfo, kept it simple.

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[am11]

cc @janvorli,

docker run -e ROOTFS_DIR=/crossrootfs/x64 -v $(pwd):/runtime \
    mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-illumos-20210714135645-f13d79e \
    /runtime/build.sh -arch x64 -os illumos -cross -gcc

(entire build) continue to succeed with this patch.

[janvorli]

The changes look good, but I'd like some libraries folks to review the related part of the change. @stephentoub could you please take a look or add someone else to review this?

[am11]

@stephentoub, @adamsitnik. the library change for official platforms has no diffs in IL or PAL binaries. I have split a partial class and updated the native implementation using existing macros, which indicates if system has a legacy struct statfs (Solaris deprecated it during the last decade) and uses stat statvfs instead which provides the filesystem name (string) instead of its ID (number). Please take a look. :)

[stephentoub]

Sorry, I missed the notification when I was tagged. Looking now...

[am11]

Windows x86 failure is unrelated (and fixed in main branch: #56649).

[adamsitnik]

System.IO part LGTM.

@am11 big thanks for another great contribution! And kudos for adding extra file system types to our existing enumeration!