arm: standalone --cortex-m internal calls need direct resolution (no linker) + $t mapping symbols #170

Follow-up to #167 (fixed for the --relocatable path in PR #169).

Standalone --cortex-m direct call resolution

The --relocatable path now emits R_ARM_THM_CALL relocations so the host linker resolves internal BL func_N calls. But a standalone --cortex-m executable has no linker — build_multi_func_cortex_m_elf must resolve internal calls directly by patching the Thumb BL offset at layout time (function addresses are known). Today those calls remain bl #0 placeholders in a standalone image.

Scope: in build_multi_func_cortex_m_elf, after laying out functions, walk each function's BL func_N relocation and patch the encoded Thumb BL displacement (S − P, accounting for the +4 pipeline) in place. The 0xF800 zero-offset encoding from PR #169 is the correct base to patch.

$t / $a / $d mapping symbols

synth's ARM objects emit no mapping symbols, so objdump won't disassemble .text as Thumb (shows .word), and strict consumers may mis-read ARM-vs-Thumb regions. Not required for linking (PR #169 links cleanly without them), but needed for clean disassembly/debug and full ARM-ELF-ABI conformance. Requires emitting a local $t at .text offset 0, which means making the .symtab sh_info (first-global index) dynamic and ordering local symbols before globals in the builder.