Don't use long double

Slots in the x87 stack get marked as occupied when you move
values into the corresponding mmx registers. If you try
to push a value onto the x87 stack, and the slot isnt empty,
it will fail, and give you a nan.

Generally, g++-x64 doesnt use the x87 instructions, but if
you do long double arithmetic, it does. f_pow was using
long doubles - but there was an alternate, much faster, asm
sequence which didnt use long double, and which just needed
to test the right define - so switch to that. Also grep
for long double to make sure we're not using it elsewhere.

hphp/runtime/base/zend/zend_multiply.h

@@ -15,7 +15,7 @@
    +----------------------------------------------------------------------+
 */
 
-#if defined(__i386__) && defined(__GNUC__)
+#if (defined(__x86_64__) || defined(__i386__)) && defined(__GNUC__)
 
 #define ZEND_SIGNED_MULTIPLY_LONG(a, b, lval, dval, usedval) do {	\
     long __tmpvar;                                                      \