tracing: Handle CC_FLAGS_FTRACE more accurately

CC_FLAGS_FTRACE is exported and later used to remove ftrace relevant
build flags from files which should be built without ftrace support.
For that reason add -mfentry to CC_FLAGS_FTRACE as well. That fixes
a problem with vdso32 build on s390, where -mfentry could not be used
together with -m31 flag.

At the same time flags like -pg and -mfentry are not relevant for asm
files, so avoid adding them to KBUILD_AFLAGS.

Introduce CC_FLAGS_USING instead of CC_USING_FENTRY to collect
-DCC_USING_FENTRY (and future alike) which are relevant for both
KBUILD_CFLAGS and KBUILD_AFLAGS.

Link: http://lkml.kernel.org/r/patch-1.thread-aa7b8d.git-42971afe87de.your-ad-here.call-01533557518-ext-9465@work.hours



Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>