Investigate building FPC trunk to enable make packages for cross-compilation

[frostney]

Context

Related to #170. The goal of replacing the curated cross-package CI bootstrap with make packages (FPC's fpmake system) was blocked by a fundamental assembler incompatibility in FPC 3.2.2.

Problem

FPC 3.2.2 generates .globl labels inside .cfi_startproc/.cfi_endproc pairs in certain units (notably rtti.pp in rtl-objpas). Xcode 16.4's Clang assembler rejects these:

non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs

This is a known FPC 3.2.2 bug that has been fixed in FPC trunk. Multiple workarounds were attempted in #217 and all failed:

• Intercepting as (PATH, -FD, compiler directory) — FPC hardcodes the SDK assembler path on macOS
• Post-hoc .s file fixing — fpmake cleans output between runs
• -s flag — leaks into fpmake bootstrap
• Label renaming / CFI stripping — either too broad (breaks .globl) or fpmake regenerates files

Proposed investigation

Evaluate building FPC from trunk (or a recent stable snapshot) instead of 3.2.2 for the cross-compilation toolchain. This would:

1. Fix the assembler label bug, enabling make packages
2. Replace the curated per-package build with automatic package installation
3. Address the original design concern from Replace curated cross-package CI bootstrap with proper FPC package installation #170 (no ad-hoc package allowlist)

Things to evaluate

• Build stability of FPC trunk for cross-compilation
• Compatibility with GocciaScript's codebase (any breaking changes vs 3.2.2)
• Whether to use trunk directly or wait for FPC 3.2.4 / 3.4.0 release
• Impact on CI cache size and build times (make packages builds ~100+ packages vs the current 4)
• Whether a smaller FPC version bump (e.g., 3.2.4 if available) fixes just this bug

Current state

The curated approach on main works and covers all packages GocciaScript currently uses:

• Compiled: rtl, rtl-objpas (4 units), rtl-generics (6 units), fcl-process (2 units)
• Source-copied: fcl-base, regexpr

There is no immediate urgency — this is a sustainability improvement.