small ABI + narrow/wide code updates

The only tangible effect this CL should have is to use __vectorcall on
all Windows builds, including scalar ones.  The code generation is a
little better there with __vectorcall than not, so might as well.  This
is a baby step towards vector stages with MSVC, but a very baby step
indeed.

Mostly this refactors and regroups a bunch of logic to reflect my
current thoughts.  The BUILD.gn changes are essentially no-ops, but they
simplify things and make our flags more similar to how those targets are
built in Chromium.

(And I cleaned up other /arch: uses so this works.)

Change-Id: I73dd39d15cdc7b3d268231a707952bbbfd91496e
Reviewed-on: https://skia-review.googlesource.com/115644
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

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