scons: When spawning the linker process, don't involve the shell.

scons: When spawning the linker process, don't involve the shell.

The command line can be too long, causing bash to choke. This means we can't
use any shell syntax like shell variables or redirection when linking, but
that should be easy to avoid.

Change-Id: Ie6c8ecab337cef6bd3c7e403346ced06f46f0993
Reviewed-on: https://gem5-review.googlesource.com/2780
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Steve Reinhardt <stever@gmail.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>