Fails to run in sandboxed environments such as Nix

[rafaelrc7]

Hello, I recently updated this package on nixpkgs (NixOS/nixpkgs#449792), and had to fix some problems with gforth. In nix packages are built inside a sandboxed environment, and one of the problems is that /sys is not available. gforth on

gforth/cilk.fs

Line 28 in f50d1cf

s" /sys/devices/system/cpu/present" slurp-file over >r

tries to access /sys/devices/system/cpu/present causing the following error:

...
warning: some-waddr is compile-only
varues.fs:107:9: warning: WA: is obsoleteEXTRADOC.FD

in file included from *args*:13:1: 
cilk.fs:28:38: error: No such file or directory
        s" /sys/devices/system/cpu/present" >>>slurp-file<<< over >r
Backtrace:
stuff.fs:171:23:                         0 $7FFFF7438DB0 perform 
*** Gforth kernel failed ***
make: *** [Makefile:1681: doc/extradoc.fd] Error 1

I fixed this issue on nixpkgs by patching gforth to call nproc (https://github.com/NixOS/nixpkgs/blob/cf3f5c4def3c7b5f1fc012b3d839575dbe552d43/pkgs/by-name/gf/gforth/use-nproc-instead-of-fhs.patch), I based the code on what cilk.fs already does for MacOS/BSD here:

gforth/cilk.fs

Lines 24 to 26 in f50d1cf

	s" sysctl -n hw.ncpu" r/o open-pipe throw slurp-fid over >r
	s>number drop
	r> free throw

While this works there, it seems to me that this is an issue where gforth assumes /sys is accessible, what may not be always true, such as in sandboxed environments such as, but not limited to, Nix. And as I believe this could be an issue on other situations I decided to open this issue.