build: Use 'always inline' macro in more places

On GCC 4.9, the inliner heuristic starts to complain again, because
SpiderMonkey 52 includes more 'always inline' functions, so when we add
ours in there, the inliner decides that functions are growing too much in
size due to inlined code.

Add a GJS_ALWAYS_INLINE macro just like Mozilla does in their code, and
use it for our templated code and other boilerplate.

https://bugzilla.gnome.org/show_bug.cgi?id=785040