Refactor simdutf into a separate composable archive to remove it as a build-matrix axis

[bartlomieju]

Problem

Every compile-time option that changes the published static library has to
become a dimension of the CI build matrix, because we ship one monolithic
librusty_v8[...]_<target>.a per feature combination. As we add more
optional features (for example the no_icu variant in #1996), the number
of archives we build and publish grows.

Some of these axes are irreducible: v8_enable_pointer_compression and
v8_enable_i18n_support are deep V8 compile flags woven through V8's core
ABI, so they fundamentally require separate full builds.

simdutf is different, and could be made composable.

Why simdutf can be separated

• simdutf is already a discrete GN static library target
  (//third_party/simdutf:simdutf).
• Only the binding functions that call into it are folded into the main
  librusty_v8.a, gated by the RUSTY_V8_ENABLE_SIMDUTF define (see the
  #ifdef RUSTY_V8_ENABLE_SIMDUTF section in src/binding.cc).

If those binding functions were compiled into their own small static
library (for example librusty_v8_simdutf.a) and published as a separate
artifact, the main archive would no longer reference simdutf at all.
Consumers that want simdutf would link the extra archive; consumers that
do not would link only the base archive. simdutf would then stop being a
matrix axis entirely.

Sketch

• Move the simdutf binding section out of src/binding.cc into its own
  translation unit compiled by a dedicated GN static_library target.
• Publish the resulting archive as a separate artifact (no _simdutf
  suffix permutations on the main archive).
• In build.rs, emit an extra cargo:rustc-link-lib/search directive for
  the simdutf archive when the simdutf feature is enabled, instead of
  selecting a different main archive.

Caveats

• This only removes the simdutf axis. ptrcomp and i18n remain separate
  full builds because they change V8's core ABI and cannot be linked in
  as add-ons.
• Needs care around symbol visibility and complete_static_lib so the two
  archives compose cleanly across platforms (including the Windows .lib).

Context: came up while adding the no_icu feature (#1996), where the slim
deno build needs simdutf + no_icu. Today that is a specific combined
archive; with this refactor it would be base + no_icu plus the standalone
simdutf archive.