Use LIBGCC_S_SO in x86_64 backtrace.

The x86_64 backtrace implementation is used as a generic
implementation (unwinding via unwind info and _Unwind_Backtrace) by
various other architectures.  This patch makes it more generic by
making it use LIBGCC_S_SO from gnu/lib-names.h instead of hardcoding
the libgcc_s.so.1 name, so that it can also be used on hppa which uses
libgcc_s.so.4.

Tested for x86_64.

	* sysdeps/x86_64/backtrace.c: Include <gnu/lib-names.h>.
	(init): Use LIBGCC_S_SO not hardcoded "libgcc_s.so.1".