Improve inlining when compiling with -Og

* src/conf_post.h (EXTERN_INLINE) [!EMACS_EXTERN_INLINE]:
Make it static inline, not merely static.  This is a worthwhile
performance improvement on my two platforms A and B (see below).
On my platform A this change improves user+system CPU performance of
‘make compile-always’ by 52% on an -Og build, and by 1.4% on the
default -O2 build.  On my platform B this improves the same benchmark
by 41% on an -Og build, and by -0.8% on the default -O2 build.
That "-0.8%" is a small negative for this change, and I recall that
it is why I didn't make this change earlier.  However, Platform B uses
an older GCC so we needn't worry overmuch about this small negative.

With this change the performance advantage of -O2 over -Og has dropped
on platform A; formerly -O2 was 87% faster than -Og, and now it is
only 25% faster.  On platform B the performance advantage of -O2 over
-Og has dropped from being 62% faster to being 14% faster.

Platform A is GCC 9.3.1 20200317 (Red Hat 9.3.1-1) on Fedora 31
x86-64 (AMD Phenom II X4 910e, circa 2010).  Platform B is GCC (Ubuntu
7.5.0-3ubuntu1~18.04) 7.5.0 on Ubuntu 18.04.4 (Intel Xeon E3-1225 V2,
circa 2012).

This patch was inspired by a suggestion by Andrea Corallo in:
https://lists.gnu.org/r/emacs-devel/2020-04/msg00263.html

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