Helps understand page switching

We have received a report of `assert!( !cb.has_dropped_bytes())` in
set_page() failing. The only explanation for this seems to be memory
allocation failing in write_byte(). The if condition implies that
`current_write_pos < dst_pos < mem_size`, which rules out failing to
encode the relative jump. The has_capacity() assert above not tripping
implies that we were in a place in the page where write_byte() did
attempt to write the byte and potentially made a syscall in the process.

Remove the assert, since memory allocation could fail. Also, return
failure if the destination is outside of the code region to detect that
out-of-memory situation quicker.

We've long had a size restriction on the code memory region such that a
u32 could refer to everything. This commit capitalizes on this
restriction by shrinking the size of `CodePtr` to be 4 bytes from 8.

To derive a full raw pointer from a `CodePtr`, one needs a base pointer.
Both `CodeBlock` and `VirtualMemory` can be used for this purpose. The
base pointer is readily available everywhere, except for in the case of
the `jit_return` "branch". Generalize lea_label() to lea_jump_target()
in the IR to delay deriving the `jit_return` address until `compile()`,
when the base pointer is available.

On railsbench, this yields roughly a 1% reduction to `yjit_alloc_size`
(58,397,765 to 57,742,248).

Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>

YJIT: Skip adding past_pages_bytes for past pages

* YJIT: Make compiled_* stats available by default

* Update comment about default counters [ci skip]

Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>

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Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>

Typo fix for the last commit (1432b37)

This is used only for arm64's cb.jmp_ptr_bytes().

* YJIT: Reduce paddings if --yjit-exec-mem-size <= 128

on arm64

* YJIT: Define jmp_ptr_bytes on CodeBlock