Reduce number of constants in __finite* (bug 15384).

Reduce number of constants in __finite* (bug 15384).

Bug 15384 notes that in __finite, two different constants are used
that could be the same constant (the result only depends on the
exponent of the floating-point representation), and that using the
same constant is better for architectures where constants need loading
from a constant pool.  This patch implements that change.

Tested for x86_64, mips64 and powerpc.

	[BZ #15384]
	* sysdeps/ieee754/dbl-64/s_finite.c (FINITE): Use same constant as
	bit-mask as in subtraction.
	* sysdeps/ieee754/dbl-64/wordsize-64/s_finite.c (__finite):
	Likewise.
	* sysdeps/ieee754/flt-32/s_finitef.c (FINITEF): Likewise.
	* sysdeps/ieee754/ldbl-128/s_finitel.c (__finitel): Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_finitel.c (__finitel): Likewise.