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".