Add an optional `alignment` argument to `Buffer.allocUnsafe()` and
`Buffer.allocUnsafeSlow()`, which guarantees that the memory backing the
returned buffer starts at an address that is a multiple of `alignment`.

Some operating system interfaces refuse to work with unaligned memory.
The motivating case is unbuffered ("direct") file I/O: a read or write
on a descriptor opened with `O_DIRECT` fails with `EINVAL` unless the
buffer address is a multiple of the logical block size of the underlying
device. Until now there was no way to obtain such a buffer from JS,
since the address of a backing store can neither be observed nor chosen.
Alignment is also worth having purely for performance, for instance to
keep a hot buffer from straddling one more cache line than its size
requires.

V8 does not allow picking the address of a backing store, so alignment
is instead achieved by over-allocating `alignment - 1` bytes and
positioning the buffer at the first suitably aligned byte within them.
The new `arrayBufferAlignedOffset()` binding computes that offset.

Addresses are not stable across snapshot serialization, so the binding
reports no padding while a snapshot is being built. Otherwise the
snapshot would capture where this particular process happened to
allocate and stop being reproducible. Buffers restored from a snapshot
are consequently not aligned; the pool works around this by recreating
itself in a deserialize callback.

The `Buffer.allocUnsafe()` pool is now aligned to a cache line itself,
which lets pooled allocations satisfy any alignment up to 64 bytes by
padding their offset into the pool rather than allocating separately.

Assisted-by: Claude/Opus 5
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Robert Nagy <ronagy@icloud.com>