Print sigfaults and exceptions to STDERR

[Sija]

While working on raven.cr I've stumbled upon the problem of intercepting exceptions and sigfaults thrown by Crystal code and to my surprise I was unable to do so. After several hours of debugging I've found the problem in using LibC.printf.

This PR changes __crystal_raise, __crystal_sigfault_handler and CallStack.print_frame to use LibC.dprintf instead.

[Sija]

Also, while sitting on it, I was wondering does [%d] notation in CallStack.print_frame shouldn't be [0x%x] like in __crystal_sigfault_handler?

[RX14]

These lines use LibC.printf instead of printf in the stdlib to avoid the nonblocking IO code. I'd suggest using LibC.dprintf with fd 2, although you'll have to bind that.

[refi64]

@RX14 do you mean fprintf?

[RX14]

fprintf expects a FILE*, while dprintf expects only a int fd, so I think the latter would be easier.

[Sija]

@RX14 thanks for the tip, I've updated the PR accordingly. I was thinking of introducing an enum (STDIN, STDOUT, STDERR) for the fd argument to improve readability, WDYT?

[bcardiff]

Thanks @Sija!

Regarding the pointers, probably the %x is better since they are expected for printing pointers. Feel free to handle that in another PR if you want, but it is not high prio.

[Sija]

I've pushed a followup #4170 PR with that change, cheers!