kbuild: create new CFLAGS_REMOVE_(basename).o option

We currently have a way to add special CFLAGS to code, but we do not have a
way to remove them if needed.

With the case of ftrace, some files should simply not be profiled. Adding
the -pg flag to these files is simply a waste, and adding "notrace" to each
and every function is ugly.

Currently we put in "Makefile turd" [1] to stop the compiler from adding -pg
to certain files. This was clumsy and awkward.

This patch now adds the revese of CFLAGS_(basename).o with
CFLAGS_REMOVE_(basename).o.  This allows developers to prevent certain
CFLAGS from being used to compile files. For example, we can now do

CFLAGS_REMOVE_string.o = -pg

to remove the -pg option from the string.o file in the lib directory.

Note: a space delimited list of options may be added to the REMOVE macro.

[1] - what David Miller called the workaronud to remove -pg
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

scripts/Makefile.lib

@@ -96,7 +96,8 @@ basename_flags = -D"KBUILD_BASENAME=KBUILD_STR($(call name-fix,$(basetarget)))"
 modname_flags  = $(if $(filter 1,$(words $(modname))),\
                  -D"KBUILD_MODNAME=KBUILD_STR($(call name-fix,$(modname)))")
 
-_c_flags       = $(KBUILD_CFLAGS) $(ccflags-y) $(CFLAGS_$(basetarget).o)
+orig_c_flags   = $(KBUILD_CFLAGS) $(ccflags-y) $(CFLAGS_$(basetarget).o)
+_c_flags       = $(filter-out $(CFLAGS_REMOVE_$(basetarget).o), $(orig_c_flags))
 _a_flags       = $(KBUILD_AFLAGS) $(asflags-y) $(AFLAGS_$(basetarget).o)
 _cpp_flags     = $(KBUILD_CPPFLAGS) $(cppflags-y) $(CPPFLAGS_$(@F))