Reimplement min_of_type using __MATH_TG.

This patch reimplements the libm-internal min_of_type macro to use
__MATH_TG instead of its own local type-generic implementation, so
simplifying the code and reducing the number of different type-generic
implementation variants in use in glibc.

Tested for x86_64.

	* sysdeps/generic/math_private.h (__EXPR_FLT128): Remove macro.
	(min_of_type_f): New macro.
	(min_of_type_): Likewise.
	(min_of_type_l): Likewise.
	(min_of_type_f128): Likewise.
	(min_of_type): Define using __MATH_TG and taking an expression
	argument.
	(math_check_force_underflow): Pass expression instead of type to
	min_of_type.
	(math_check_force_underflow_nonneg): Likewise.