Objective-C/C++ (C-family) : Add missing 'atomic' property attribute.

This is the default, but it is still legal in user code and therefore
we should handle it in parsing.  Fix whitespace issues in the lines
affected.

gcc/c-family/ChangeLog:

	* c-common.c (c_common_reswords): Add 'atomic' property
	attribute.
	* c-common.h (enum rid): Add RID_PROPATOMIC for atomic
	property attributes.

gcc/objc/ChangeLog:

	* objc-act.c (objc_prop_attr_kind_for_rid): Handle
	RID_PROPATOMIC.

gcc/testsuite/ChangeLog:

	* obj-c++.dg/property/at-property-4.mm: Test atomic property
	attribute.
	* objc.dg/property/at-property-4.m: Likewise.