x86: Set the effective base of the TSS when initializing a process.

x86: Set the effective base of the TSS when initializing a process.

For some segments, there are two base registers. One is the
architecturally visible base, and the other is the effective base used
when actually referencing memory relative to that segment. The process
initialization code was setting the architecturally visible base,
presumably because that's the value used by KVM, but was setting the
effective base to zero.

Change-Id: I06e079f24fa63f0051268437bf00c14578f62612
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38488
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>