Fix a regression introduced in r274337 (large block support)

In dsl_dataset_hold_obj() we used zap_contains(.., DS_FIELD_LARGE_BLOCKS)
to determine whether the extensible (zapifyed) dataset have large blocks.
The code expects the result be either 0 (found) or ENOENT (not found),
however reused the variable 'err' which later code expects to be 0.

Fix this by adopting similar code construct that is used later for
DS_FIELD_BOOKMARK_NAMES, which uses a temporary variable zaperr to catch
errors from zap_* rountines.

Reported by:	Peter J. Creath (on FreeNAS; FreeNAS bug #6848)
Illumos issue:	5393 spurious failures from dsl_dataset_hold_obj()
Reviewed by:	mahrens
Sponsored by:	iXsystems, Inc.
X-MFC with:	r274337
Update the constants associated with the evaluation of j0f(x)
for |x| small.

While here, remove the explicit cast of 0.25 to float.  Replace
a multiplication involving 0.25 by a division using an integer
constant 4.  Make a similar change in j0() to minimize the diff.

Suggested by:	bde
Add missing END macros to some of the xscale functions.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Use the unified syntax in a few more assembly files

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Set the alignment to 4-bytes after a string as clang 3.5 can switch to
thumb mode if this is incorrect.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Place the literal pool after a RET otherwise clang 3.5 tries to put it too
far away from a ldr psuedo instruction. With this clang will place the
literal value here where it's close enough to be loaded.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Switch to an armv6k cpu, without this clang 3.5 complains "bx lr" is
unsupported as it needs a newer cpu.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Switch to a .cpu directive. These will work when clang 3.5 is imported
where the .arch directive is a nop.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd
Use %d instead of %u for error number.  This way we see ERESTART as -1
not 4294967295 when doing DTrace.

MFC after:	2 weeks