Revision 1909321

atomics: Disentangle 32bit and 64bit atomics configuration.

Regardless of --enable-nonportable-atomics setting, 64bit atomic builtins are
disabled for 32bit CPUs/systems (using the generic/mutex implementation), until
we figure out how to properly align apr_uint64_t (at least i[56]86 CPUs which
could work with 64bit builtins actually don't if the data is not 64bit aligned,
assuming the same for other 32bit CPUs is safer).

One can --enable-nonportable-atomics=upto32bit to explicitely enable 32 bit
builtins only (if available in the first place).

Using --enable-nonportable-atomics=no still disables both 64bit and 32bit
builtins, while =yes enables the ones known to exist AND work only (i.e. both,
32bit or none).

Rename NEED_ATOMICS_GENERIC64 to USE_ATOMICS_GENERIC64 for consitency with
32bit atomics.