Clean-up nolock.h: it doesn't serve any purpose anymore. Appropriate code moved
to x86-gcc.h and my_atomic.h.

If gcc sync bultins were detected, we want to make use of them independently of
__GNUC__ definition. E.g. XLC simulates those, but doesn't define __GNUC__.

HS/Spider: According to AIX manual alloca() returns char*, which cannot be
casted to any type with static_cast. Use explicit cast instead.

MDL: Removed namemangling pragma, which didn't let MariaDB build with XLC.

WSREP: _int64 seem to be conflicting name with XLC, replaced with _integer64.

CONNECT: RTLD_NOLOAD is GNU extention. Removed rather meaningless check if
library is loaded. Multiple dlopen()'s of the same library are permitted,
and it never gets closed anyway. Except for error, which was a bug: it may
close library, which can still be referenced by other subsystems.

InnoDB: __ppc_get_timebase() is GNU extention. Only use it when __GLIBC__ is
defined.

Based on contribution by flynn1973.