ARM: 8158/1: LLVMLinux: use static inline in ARM ftrace.h

ARM: 8158/1: LLVMLinux: use static inline in ARM ftrace.h

With compilers which follow the C99 standard (like modern versions of gcc and
clang), "extern inline" does the wrong thing (emits code for an externally
linkable version of the inline function). In this case using static inline
and removing the NULL version of return_address in return_address.c does
the right thing.

Signed-off-by: Behan Webster <behanw@converseincode.com>
Reviewed-by: Mark Charlebois <charlebm@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[Backport from mainline to fix build error with gcc 5.x or later]
Reported-by: Sung-jae Park <nicesj@nicesj.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I8a1cbff2ce69a9d9d021b434ae062fdf0bbc67c0