Do not allocate memory in the segmentation fault signal handler

[HertzDevil]

The String.new(UInt8*) call inside Exception::CallStack.decode_frame could allocate memory, potentially triggering a GC cycle or expanding the GC heap, which could break the stack trace:

x = Pointer(UInt8).null
y = x.value

Invalid memory access (signal 11) at address 0x0
[0x55dbcfe66a06] *Exception::CallStack::print_backtrace:Nil +118 in /home/quinton/.cache/crystal/crystal-run-test.tmp
[0x55dbcfe55ec6] Program exited because of an invalid memory access

This seems to happen on master since #14257, and also to #14047 with Crystal 1.10.1. This PR removes this allocation so that Crystal::System::Signal.@@segfault_handler should be allocation-free. (Technically Fiber.current could eventually call Thread.new, but this should be extremely rare.)

[straight-shoota]

Perhaps you could add a comment about that to the Fiber.current call as well?