Handle always_inline as GNU inline

Handle always_inline as GNU inline

this is needed for multi-file testcases using stdio.h, as
the __sputc function is implemented as a extern inline
function (with gnu_inline attribute, but we don't support that for now).

Without this change that leads to multiply defined symbols when using
multiple units including stdio.h.

It also has an always_inline attribute, which we can use to guide our
behaviour, as in ISO-C an always_inline can't be defined with ISO
'extern inline' semantics.  This is the minimal change and not a full
implementation of GNU inline semantics, which would require thorough
testcases.

If __clang__ would be defined the header would make use of C99 semantics,
which would work for us.  It would also do that if _GNUC_ wouldn't be
defined.  But we can't do the latter (as the whole MacOSX SDK refuses
to be compiled with anything not defining that).  I haven't tested
defining __clang__, but suspect that's going to be problematic.