Make ldexpF generic.

This one is a little more tricky since it is built both for
libm and libc, and exports multiple aliases.

To simplify aliasing, a new macro is introduced which handles
aliasing to two symbols.  By default, it just applies
declare_mgen_alias to both target symbols.

Likewise, the makefile is tweaked a little to generate
templates for shared files too, and a new rule is added
to build m_*.c objects from the objpfx directory.

Verified there are no symbol or code changes using a script
to diff the *_ldexp* object files on s390x, aarch64, arm,
x86_64, and ppc64.