Add exp10 macro to <tgmath.h> (bug 26108)

Add exp10 macro to <tgmath.h> (bug 26108)

glibc has had exp10 functions since long before they were
standardized; now they are standardized in TS 18661-4 and C2X, they
are also specified there to have a corresponding type-generic macro.
Add one to <tgmath.h>, so fixing bug 26108.

glibc doesn't have other functions from TS 18661-4 yet, but when
added, it will be natural to add the type-generic macro for each
function family at the same time as the functions.

Tested for x86_64.