Convert e_exp2l.c into a template

Convert e_exp2l.c into a template

This patch converts the implementation of exp2l in math/e_exp2l.c into
a template in math/e_exp2_template.c, then adjusts Makefile to use
this template for long double (the implementations for float and
double in sysdeps have higher precedence and are not used).  This
template can also be used for float128, thus reducing the amount of
duplicated code that gets added when adding support the new type.

Tested for powerpc64le and s390x.

	* math/Makefile (libm-calls): Move e_exp2F to gen-libm-calls.
	(gen-libm-calls): Add e_exp2F to use the template.
	* math/e_exp2l.c: Rename to math/e_exp2_template.c.
	* math/e_exp2_template.c: New file, renamed from
	math/e_exp2l.c, and made into a template.
	* sysdeps/generic/math-type-macros.h (M_MIN_EXP): New macro.