Use LOAD_PIC_REG in i386 atanh.

sysdeps/i386/fpu/e_atanh.S, unlike all other functions in that
directory, loads the PIC register with its own code using
_GLOBAL_OFFSET_TABLE_, rather than with the LOAD_PIC_REG macro.  I see
no good reason for the difference; this patch makes it use the common
macro.

Tested for x86.

	* sysdeps/i386/fpu/e_atanh.S (__ieee754_atanh) [PIC]: Use
	LOAD_PIC_REG.