scons: fix --gold-linker build after --as-needed

scons: fix --gold-linker build after --as-needed

The build was failing with:

/usr/bin/ld: unrecognized option '--as-needed -fuse-ld=gold'

and --verbose confirms that a single quoted CLI parameter was being
executed:

"-Wl,--as-needed -fuse-ld=gold"

This happened because at Ifb001786a66b0dd9b29865e39a5740313002f250
--as-needed was added, and because it is the second option to happen before
the following main.subst, it exposed the fact that the existing main.subst
was wrong, because it returns a string instead of the expected array.

Change-Id: I619d242d60fe9d27438638ac11c2b92512881f26
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24624
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>