scons: Fix 'CheckLinkFlag' so '-Werror' is not enabled
scons: Fix 'CheckLinkFlag' so '-Werror' is not enabled

A small error in 'CheckLinkFlag' was resulting in `-Werror` being set
as a CXXFLAG after the function was run. This patch fixes this.

Change-Id: I786f81a4f94be0356573eb6d4673d04ee5aa844d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54863
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5