libphobos: Add @nogc to gcc.backtrace and gcc.libbacktrace modules.

libphobos: Add @nogc to gcc.backtrace and gcc.libbacktrace modules.

libphobos/ChangeLog:

	* libdruntime/gcc/backtrace.d (simpleErrorCallback): Add @nogc.
	(LibBacktrace.initLibBacktrace): Likewise.
	(LibBacktrace.this): Likewise.
	(UnwindBacktrace.this): Likewise.
	(getBacktrace): Likewise.
	(getBacktraceSymbols): Likewise.
	* libdruntime/gcc/libbacktrace.d.in (backtrace_create_state):
	Likewise.
	(backtrace_full): Likewise.
	(backtrace_simple): Likewise.
	(backtrace_print): Likewise.
	(backtrace_pcinfo): Likewise.
	(backtrace_syminfo): Likewise.