cdefs.h: add inline to always_inline

Add inline to the always_inline macro to quiet dozens of gcc
warnings of the form:
warning: 'always_inline' function might not be inlinable [-Wattributes]

It's clearly the intention of the always_inline macro applied to a
function to inline the function. However, gcc seems to be picky with
the -Wattributes. It appears that attribute((always_inline))
was intended to apply to inline functions, as in, function declarations
with the attribute should also be declared as inline. Both clang and
gcc sources themselves use the two in combination:
inline attribute((always_inline__))

FreeBSD sources mostly only use __always_inline, without the inline
keyword. Only a few files in libmsun used both.

Reviewed by: imp, kib, olce
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45711