build: Don't redefine 'inline'

Actively redefining 'inline' is wrong for C++, where gcc has an
extension 'inline namespace' which fails to compile if the
keyword 'inline' is replaced by a macro expansion.  This will
matter once we start to include "qemu/osdep.h" first from C++
files, depending also on whether the system headers are new
enough to be using the gcc extension.

But rather than just guard things by __cplusplus, let's look at
the overall picture.  Commit df2542c737ea2 in 2007 defined 'inline'
to the gcc attribute __always_inline__, with the rationale "To
avoid discarded inlining bug".  But compilers have improved since
then, and we are probably better off trusting the compiler rather
than trying to force its hand.

So just nuke our craziness.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1455043788-28112-1-git-send-email-eblake@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
