pragma: Update target option node when optimization changes [PR103515]

For a function with optimize pragma, it's possible that the target
options change as optimization options change.  Now we create one
optimization option node when optimize pragma parsing, but don't
create target option node for possible target option changes.  It
makes later processing not detect the target options can actually
change and further doesn't update the target options accordingly.

This patch is to check whether target options have changed when
creating one optimization option node for pragma optimize, and
make one target option node if needed.  The associated test case
shows the difference.  Without this patch, the function foo1 will
perform unrolling which is unexpected.  The reason is that flag
unroll_only_small_loops isn't correctly set for it.  The value
is updated after parsing function foo2, but doesn't get restored
later since both decls don't have DECL_FUNCTION_SPECIFIC_TARGET
set and the hook thinks we don't need to switch.  With this patch,
there is no unrolling for foo1, which is also consistent with the
behavior by replacing pragma by attribute whether w/ and w/o this
patch.

As Martin noted, this change does the similar thing like what his
previous commit r12-1039 did.

gcc/ChangeLog:

	PR target/103515
	* attribs.c (decl_attributes): Check if target options change and
	create one node if so.

gcc/testsuite/ChangeLog:

	PR target/103515
	* gcc.target/powerpc/pr103515.c: New test.