Fix build issue on macOS (aarch64, gcc) and use debugtrap wherever possible

[MrAnno]

This PR makes debugbreak prefer __builtin_debugtrap() wherever it's available (clang).

The code snippet I've used is @jwakely's work, I've added a signoff to my commit message to mark the original author.

The patch fixes a build issue too on macOS (Apple silicon) when a real GCC compiler is used:

warning: implicit declaration of function '__builtin_debugtrap';

Depends on #22
Fixes #24

---

I'm testing this change on the following architectures (with clang + gcc):

• x86-64
  • gcc/clang: works
• ARM mode, 32-bit (Raspberry Pi 1 armv6l)
  • gcc/clang: works (stepping in GDB does NOT work)
• Thumb mode, 32-bit (Cirrus CI)
  • gcc/clang: works (stepping in GDB does NOT work)
• AArch64, ARMv8 (Raspberry Pi 4)
  • gcc/clang: works (stepping in GDB does NOT work)
• Apple silicon (macOS M1 machine):
  • gcc: works (stepping in LLDB does NOT work)
  • clang: works

Unfortunately, I won't be able to test the change on riscv and powerpc.

[jwakely]

I'll see if I can test it on powerpc64.

[MrAnno]

I've completed my tests on the platforms that were available to me.
It seems nothing got worse with this change, just slightly better, for example, on macOS.

@scottt What do you think? Is this something you would consider integrating into the master branch?

[jwakely]

on POWER8 running CentOS 7.9:

$ test/fib ; test/trap ; test/break ; test/break-c++
Trace/breakpoint trap (core dumped)
Trace/breakpoint trap (core dumped)
Trace/breakpoint trap (core dumped)
Trace/breakpoint trap (core dumped)

[MrAnno]

@jwakely Thank you.

[MrAnno]

@scottt Could you have a look at this?

[MrAnno]

We had to start using a fork of debugbreak due to this build issue on macOS.

[MrAnno]

@scottt Do you have any comments on this?