jsapi-util-args: Mark functions as always-inline

On GCC, we get warnings about the inline functions causing too much code
growth. However, we do want these functions always inlined, since they're
in a header file.

Previously we dealt with the warnings by increasing --param
inline-unit-growth, but that causes other warnings on Clang since Clang
doesn't have that parameter. I also noticed that it requires different
values on other versions of GCC, so it's not a very good solution.

Instead, we use the always_inline attribute on GCC, which causes the
functions to be inlined regardless of the compiler's inlining heuristics.

https://bugzilla.gnome.org/show_bug.cgi?id=780350