Add opt-in register-allocation tracing (AGBCC_TRACE_ALLOC)

[SensanaMMZ]

Adds an opt-in diagnostic that makes the register allocator explain its decisions, which is very useful when reconstructing C for a byte-match and a value lands in an unexpected register.

When the AGBCC_TRACE_ALLOC environment variable is set, the compiler emits one line per hard-register assignment to stderr:

[lalloc] pseudo 42 -> r0  (qty 0, births 2, deaths 26, n_calls_crossed 0)
[galloc] pseudo 39 -> SPILL (allocno 9, calls_crossed 0)
[galloc] pseudo 39 -> r12  (allocno 9, size 1, calls_crossed 0, live_length 24, n_refs 3)

The numbers are exactly the priority inputs the allocator sorts on, so you can see why a pseudo lost the low-register race and got retried into a high register, rather than inferring it from the output.

• 21 lines across gcc/global.c (the find_reg commit and spill paths) and gcc/local-alloc.c (quantity propagation).
• Entirely gated on the environment variable; with it unset the compiler is unchanged. Verified that codegen is byte-identical with tracing off and on.
• No new build flags, no header changes.