e_os.h: allow inline functions to be compiled by legacy compilers.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 40155f408985aff2e9f1b61b7cb04a3e518633a1)
e_os.h: refine inline override logic (to address warnings in debug build).

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
(cherry picked from commit 55c7a4cf112bf154ed405ee05a6b7924b6b1ba92)