@haypo: Yes, that patch works (applied to 3.5.0rc3; proxy problems are preventing me from updating my clone of the default branch at the moment) i.e., pyconfig.h shows:

#define HAVE_GETRANDOM_SYSCALL 1

To answer your other questions:
* Calling syscall(SYS_getrandom, buffer, size, 0) does work on Solaris.
* Our <sys/syscall.h> does declare syscall().
* Solaris does not use the GNU C library, but our own libc, which does
  support getrandom(3) as of Solaris 11.3 (which will be released in
  the near future) and Solaris 12 (which is in Beta but has a release
  date in the more distant future).

Prior to applying this patch, I had needed to tweak py_getrandom() to recognize EINVAL in addition to ENOSYS as sufficient reason to turn off getrandom_works.  I still have that tweak in place, but have not yet tried backing it out to see if things still behave with your patch.

Let me know if there is any more information I can provide, and thanks for your attention to this issue.