build: Allow more compiler inlining

jsapi-util-args.h relies on inlining a lot of functions, into functions
that are relatively small and therefore grow a lot. This started to cause
warnings when switching to SpiderMonkey 38 (presumably because more
methods in SpiderMonkey headers are inlined as well.)

The default value for inline-unit-growth is 30, which according to the
GCC documentation allows functions to grow by 30% as a result of
inlining. Presumably this empirically chosen value of 50 will allow them
to grow by 50%.

https://bugzilla.gnome.org/show_bug.cgi?id=776966