Change return type in the declaration of __ieee754_rem_pio2l

The implementation of __ieee754_rem_pio2l in ldbl-128, ldbl-128ibm,
and ldbl-96 return the type int32_t, whereas math_private.h declares
it as returning int.  This patch changes the declaration to match the
declaration in thoses directories, as well as it changes the stub
implementation in math/e_rem_pio2l.c, similarly.

	* math/e_rem_pio2l.c (__ieee754_rem_pio2l): Change return type
	to int32_t.
	* sysdeps/generic/math_private.h: Declare __ieee754_rem_pio2l
	as returning int32_t.