staticdata: Refactor sysimage loading (#57542)
Introduce `jl_image_buf_t` to represent the in-memory details of an
unparsed system image and clean-up several global variables and split
loading logic in staticdata.c so that we get (almost) everything we need
from the sysimage handle at once.
This allows sysimage loading to be separated into three phases:
1. Lookup the raw sysimage buffer as a `jl_image_buf_t`
2. For .so sysimages, parse the sysimage and initialize JIT targets,
etc.
3. Finally load the sysimage into a `jl_image_t`
Care was taken to preserve the existing behavior of calling
`jl_set_sysimg_so` to configure the sysimage before initializing Julia,
although this is likely to be next on the chopping block in a follow-up.
Dependent on #57523.
