scons: Remove the SPAWN hack added earlier.

scons: Remove the SPAWN hack added earlier.

A previous change forced scons to spawn child processes by exec-ing it
directly rather than going through the shell because the command line
length would be too long for the shell to handle. Now that incremental
linking should keep the command line lengths more under control, that
change should no longer be necessary.

Change-Id: I9e82a62083afd1414324a7fd697bd6d4b76367ae
Reviewed-on: https://gem5-review.googlesource.com/2947
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>