Add
--bp-compression-sort-section=<glob>[=<layout_priority>[=<match_priority>]]
to let users split input sections into multiple compression groups, run
balanced partitioning independently per group, and leave out sections
that are poor candidates for BP. This replaces the old coarse
--bp-compression-sort with a more explicit, user-controlled one.
In ELF, the glob matches input section names (.text.unlikely.cold1). In
Mach-O, it matches the concatenated segment+section name (__TEXT__text).
layout_priority controls group placement in the final layout.
match_priority resolves conflicts when multiple globs match the same
section: explicit priority beats positional matching, and among
positional specs the last match wins.
A CRTP hook getCompressionSubgroupKey() allows backends to further
subdivide glob groups into independent BP instances. This allows Mach-O
backend to separate cold functions via N_COLD_FUNC in the future.
The deprecated --bp-compression-sort option keeps its existing
function/data behavior by assigning sections to fixed legacy groups.
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