Fix breakpoints on macOS to trap EXC_BREAKPOINT with correct
source location when using lldb. Old behavior with `raise(SIGTRAP)`
traps SIGTRAP and incorrect source location.

Fix breakpoints on archlinux to trap SIGILL with correct source
location when using gdb. Old behavior with `raise(SIGTRAP)`
traps SIGTRAP and (sometimes) incorrect source location with
very shallow (break in main) stack.

when building stage1:
- w/ clang, use `__builtin_debugtrap()`
- w/ gcc, use `__builtin_trap()`
- else use `raise(SIGTRAP)`