kbuild: fix ia64 breakage after introducing make -rR

kbuild used $¤(*F to get filename of target without extension.
This was used in several places all over kbuild, but introducing
make -rR broke his for all cases where we specified full path to
target/prerequsite. It is assumed that make -rR disables old style
suffix-rules which is why is suddenly failed.

ia64 was impacted by this change because several div* routines in
arch/ia64/lib are build using explicit paths and then kbuild failed.

Thanks to David Mosberger-Tang <David.Mosberger@acm.org> for an explanation
what was the root-cause and for testing on ia64.

This patch also fixes two uses of $(*F) in arch/um

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1352,7 +1352,7 @@ quiet_cmd_rmfiles = $(if $(wildcard $(rm-files)),CLEAN   $(wildcard $(rm-files))
 
 a_flags = -Wp,-MD,$(depfile) $(AFLAGS) $(AFLAGS_KERNEL) \
 	  $(NOSTDINC_FLAGS) $(CPPFLAGS) \
-	  $(modkern_aflags) $(EXTRA_AFLAGS) $(AFLAGS_$(*F).o)
+	  $(modkern_aflags) $(EXTRA_AFLAGS) $(AFLAGS_$(basetarget).o)
 
 quiet_cmd_as_o_S = AS      $@
 cmd_as_o_S       = $(CC) $(a_flags) -c -o $@ $<