Adds an ArrayBuffer-based invocation path for FFI functions whose
signatures are composed entirely of numeric types (i8..i64, u8..u64,
f32, f64, bool, char) and/or pointer types. The JS wrapper packs
arguments directly into a per-function AB via primordial DataView
setters and the C++ invoker (InvokeFunctionSB) reads them without
going through V8's FunctionCallbackInfo. Results are returned the same
way.

Pointer arguments use runtime dispatch: BigInt, null, and undefined take
the fast path, while Buffer, ArrayBuffer, ArrayBufferView, and String
fall back transparently to the classic InvokeFunction path via a
stashed _invokeSlow function. Signatures containing
non-numeric/non-pointer types also bypass the fast path.

The fast path is disabled on big-endian platforms.

Callers do not opt in, and the fast path is transparent in every way
users should rely on. One observable change: function wrappers returned
by library.getFunction, library.getFunctions, and library.functions
now have .length equal to the declared parameter count rather than
0. Code that relied on the previous value will need to be updated.

This is largely based on #46905, which is largely based on https://github.com/bengl/sbffi.

Benchmark output:

                                       confidence improvement accuracy (*)   (**)  (***)
ffi/add-f64.js n=10000000                     ***     28.58 %       ±0.83% ±1.12% ±1.47%
ffi/add-i32.js n=10000000                     ***     24.01 %       ±0.77% ±1.02% ±1.33%
ffi/getpid.js n=10000000                               0.17 %       ±1.26% ±1.69% ±2.24%
ffi/many-args.js n=10000000                   ***    -11.76 %       ±0.61% ±0.81% ±1.06%
ffi/pointer-bigint.js n=10000000              ***     53.44 %       ±1.27% ±1.69% ±2.20%
ffi/sum-buffer.js n=1000000 size=1024         ***     16.97 %       ±0.78% ±1.04% ±1.35%
ffi/sum-buffer.js n=1000000 size=16384        ***      2.02 %       ±0.70% ±0.94% ±1.23%
ffi/sum-buffer.js n=1000000 size=64           ***     26.10 %       ±0.63% ±0.83% ±1.09%

TODO:

• rebase