alpha: math-emu: Fix modular build

Commit 00fc0e0d ("alpha: move exports to actual definitions") also
removed the exports of the math emulator hooks, which are defined in C
code.  In case anyone cares about the option of CONFIG_MATHEMU=m, add
exports next to those definitions.  Also add a MODULE_LICENSE.

Fixes: 00fc0e0d

 ("alpha: move exports to actual definitions")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>