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This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.

The allocation functions no longer assume that one RVALUE needs to be
allocated.

ast.c: Use kept script_lines data instead of re-open the source file

As noticed by @nobu https://github.com/rubygems/rubygems/pull/4989#discussion_r735674633

From wikipedia: https://en.wikipedia.org/wiki/SHA-1#SHA-1_pseudocode

> append ml, the original message length in bits, as a 64-bit big-endian integer.

`Q` is native endian, so little-endian on most modern hardware.
The original code from RubyDigest reverses the bytes:
https://github.com/Solistra/ruby-digest/blob/d15f906caf09171f897efc74645c9e31373d7fd1/lib/ruby_digest.rb#L521

But that makes the code non-portable, the correct way is to directly ask
for a big-endian representation.

https://github.com/rubygems/rubygems/commit/ba2be01ea4

In an effort to minimize build issues on non x64 platforms, we can
decide at build time to not build the bulk of YJIT. This should fix
obscure build errors like this one on riscv64:

    yjit_asm.c:137:(.text+0x3fa): relocation truncated to fit: R_RISCV_PCREL_HI20 against `alloc_exec_mem'

We also don't need to bulid YJIT on `--disable-jit-support` builds.

One wrinkle to this is that the YJIT Ruby module will not be defined
when YJIT is stripped from the build. I think that's a fair change as
it's only meant to be used for YJIT development.