x86: Simplify and consolidate the code that assembles an MSI on x86.

x86: Simplify and consolidate the code that assembles an MSI on x86.

There is no interrupt response message, and so no need for a function
which would construct one. The other functions which construct the
request can be consolidated since the work being done by each is
incremental. The template parameters can be used to support multiple
types and offsets in a single function, and since that function also
doesn't have to do much work, it makes sense to do everything in one
shot.

Change-Id: I41b202a263a697c5ada6817f3ab2a4728281b894
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20826
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>