perf tools: Fix x86 builds with ARCH specified on the command line

e.g., compiling i386 on x86_64 using:
$ make -C tools/perf ARCH=i386

fails with:

    CC /tmp/pbuild/util/evsel.o
In file included from util/evsel.c:21:0:
util/perf_regs.h:5:23: fatal error: perf_regs.h: No such file or directory
compilation terminated.

Adding V=1 you see that the include argument for the arch is
'-Iarch/i386/include' is wrong. It is supposed to be -Iarch/x86/include
per the redefinition of ARCH in the Makefile.

According to the make manual,
http://www.gnu.org/software/make/manual/make.html#Override-Directive:
  "If a variable has been set with a command argument (see Overriding
   Variables), then ordinary assignments in the makefile are ignored. If
   you want to set the variable in the makefile even though it was set
   with a command argument, you can use an override directive ..."

Make it so.
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1346094354-74356-1-git-send-email-dsahern@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile

@@ -64,12 +64,12 @@ AR = $(CROSS_COMPILE)ar
 
 # Additional ARCH settings for x86
 ifeq ($(ARCH),i386)
-	ARCH := x86
+	override ARCH := x86
 	NO_PERF_REGS := 0
 	LIBUNWIND_LIBS = -lunwind -lunwind-x86
 endif
 ifeq ($(ARCH),x86_64)
-	ARCH := x86
+	override ARCH := x86
 	IS_X86_64 := 0
 	ifeq (, $(findstring m32,$(EXTRA_CFLAGS)))
 		IS_X86_64 := $(shell echo __x86_64__ | ${CC} -E -xc - | tail -n 1)