Exclude signal handler from linux crashlogs

[NicksWorld]

Currently all crashlogs produced on Linux end with a opaque offset within libdfhack.so, which can cause some confusion.

Signal SIGSEGV
0> hack/libdfhack.so(+0x936f90) [0x7b040d336f90]

This PR causes the crashhandler to skip printing the first backtrace entry, which will always be dfhack_crashlog_handle_signal or dfhack_crashlog_handle_terminate, where the backtrace is retrieved. To ensure exactly one crashhandler-related entry is added to the stack frames, handle_signal_internal is marked with [[gnu::always_inline]] to ensure it doesn't create a stack frame.

Alternatively, we can clear up the opaque entry by enabling symbol visibility on dfhack_crashlog_handle_(signal|terminate) by marking them with DFHACK_EXPORT, with the following result:

Signal SIGSEGV
0> hack/libdfhack.so(dfhack_crashlog_handle_signal+0x62) [0x7fffe3955052]