In low memory situations mmap(2) on Illumos[2] can return EAGAIN when it
is unable to reserve the necessary space for the requested mapping.  Go
was not previously handling this correctly for Illumos and would fail to
recognize it was in a low-memory situation, the result being the program
would terminate with a panic instead of running the GC.

Fixes: #14930

[1]: https://www.illumos.org/man/2/mmap

Change-Id: I889cc0547e23f9d6c56e4fdd7bcbd0e15403873a
Reviewed-on: https://go-review.googlesource.com/43461Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>