Fix and code refactoring of lock elision workaround (Linux/x64)

Fix and code refactoring of lock elision workaround (Linux/x64)

* configure.ac (HAVE_LIBC_VERSION_H, HAVE_GNU_GET_LIBC_VERSION): Remove
(revert change in previous commit).
* include/private/gcconfig.h (GLIBC_2_19_TSX_BUG): New macro defined
for Linux/x86_64 (if Glibc used) to workaround a bug in Glibc lock
elision implementation.
* pthread_support.c: Move include of gnu/libc-version.h to gcconfig.h
(used to check whether lock elision workaround needed).
* misc.c (GC_init): Reformat code.
* pthread_support.c (mark_mutex): Initialize (to
PTHREAD_MUTEX_INITIALIZER) even lock elision workaround is needed
(revert change in previous commit).
* pthread_support.c (parse_version): New static function (defined only
if GLIBC_2_19_TSX_BUG).
* pthread_support.c (GC_setup_mark_lock): Use parse_version to check
target Glibc version properly; do not reinitialize mutex unless
workaround needed; call ABORT (with the appropriate message) in case
of a failure in pthread_mutexattr_init/settype, pthread_mutex_init.