Add register parameter support for m68k-amiga-elf GCC (gcc-regparm.patch)

[jbilander]

This patch adds support for explicit asm("reg") /__asm__("reg") register
parameter constraints on function declarations, enabling the AmigaOS/vbcc-style
calling convention directly in C:

void DevOpen(struct Library *dev asm("a6"),
struct IORequest *ioreq asm("a1"),
ULONG unit asm("d0"),
ULONG flags asm("d1"));

Both asm() and __asm__() syntax are accepted.

Motivation

The current toolchain requires inline asm macro stubs for all AmigaOS library
and device driver code that uses register parameters. This patch allows writing
device drivers, library stubs and ROM-callable code in natural C with the same
syntax supported by vbcc, SAS/C and Bebbo's amiga-gcc.

Changes

Five files in GCC 15.1.0 are modified (applied after gcc-barto.patch):

• gcc/config/m68k/m68k.h — FUNCTION_ARG_REGNO_P, CUMULATIVE_ARGS struct
• gcc/config/m68k/m68k.cc — m68k_get_parm_regno_at, m68k_function_arg,
  m68k_function_arg_advance
• gcc/c/c-tree.h — asm_name field in struct c_parm
• gcc/c/c-decl.cc — build_c_parm, grokparm, push_parm_decl
• gcc/c/c-parser.cc — parse asm() after parameter declarator

Testing

Verified on Linux Mint 22 building GCC 15.1.0 from source, and confirmed
working on real AmigaOS hardware in WinUAE:

• https://github.com/jbilander/amiga-regparm-test — minimal CLI test program
• https://github.com/jbilander/SimpleDevice — Amiga device driver using register parameters

The approach is based on the same mechanism used by Bebbo's amiga-gcc
(amiga13.x/amiga15.x branches), ported and adapted for this toolchain's
GCC 15.1 base and the A5 frame pointer convention introduced by gcc-barto.patch.

[jbilander]

Note: This PR was opened from my fork's master branch which contains
additional work (NDK 3.2R4 headers, build documentation) that is
covered by a separate PR #300. The only change relevant to this PR
is gcc-regparm.patch. Sorry for the noise!

[jbilander]

Update: field testing in a real device driver (par2ser.device) uncovered
that asm("reg") annotations on prototypes were silently ignored at call
sites — the caller used the stack ABI while the callee read registers,
corrupting memory. Fixing that properly surfaced three deeper issues in
the original implementation, all resolved in the latest commit:

1. Annotated prototypes/typedefs/function pointers are now a hard
   compile error ("register parameter 'x' is only supported on function
   definitions") instead of a silent runtime trap.
2. The register map is now carried as internal decl attributes on the
   FUNCTION_DECL rather than via DECL_ASSEMBLER_NAME on PARM_DECLs.
   PARM_DECLs have no assembler-name field (the old approach aliased
   DECL_INCOMING_RTL and would ICE on a checking-enabled GCC), and
   cgraph frees DECL_ARGUMENTS after compiling a function, which made
   same-TU callers silently fall back to the stack ABI.
3. Sibcalls to register-parameter functions are disabled: the sibcall
   epilogue restores call-saved registers (d2-d7/a2-a5) after the
   outgoing arguments are loaded, clobbering them; a normal jsr is
   always safe.

Supported scope: register parameters on definitions, callable from
OS/asm vectors and from C within the same translation unit; annotated
prototypes are rejected. Cross-TU prototype support (register list as a
function type attribute, as Bebbo's gcc does) remains future work.

Regression tested by rebuilding four real projects against the updated
compiler and inspecting the disassembly of each: amiga-regparm-test,
SimpleDevice, hello-world-amiga-gcc (NDK 3.2R4 proto/ headers,
correct jsr -N(a6) library calls), and par2ser.device (4 TUs + asm).
Callee-side codegen is unchanged; the same-TU caller test now emits
jsr with d2/d3/d4 loaded and no stack pushes; the negative prototype
test errors as expected. Tests added to
https://github.com/jbilander/amiga-regparm-test