kexec broken with STACKLEAK enabled

[Arvind Sankar]

Hi, since commit b059f801a937 (x86/purgatory: Use CFLAGS_REMOVE rather
than reset KBUILD_CFLAGS) kexec is broken if GCC_PLUGIN_STACKLEAK is
enabled, as the purgatory contains undefined references to
stackleak_track_stack.

Attempting to load a kexec kernel results in an error:

	kexec: Undefined symbol: stackleak_track_stack
	kexec-bzImage64: Loading purgatory failed

Adding $(DISABLE_STACKLEAK_PLUGIN) to PURGATORY_CFLAGS in
arch/x86/purgatory/Makefile fixes this.

Not sure if that's the best fix or if other architectures also require a
similar one.

Thanks.