Add OsX ppc compilation support.

From upstream (24c2fa1245)

Now compiles without error on a G4 processor.

c_src/double-conversion/utils.h

@@ -55,7 +55,8 @@
 #if defined(_M_X64) || defined(__x86_64__) || \
     defined(__ARMEL__) || defined(__avr32__) || \
     defined(__hppa__) || defined(__ia64__) || \
-    defined(__mips__) || defined(__powerpc__) || \
+    defined(__mips__) || \
+    defined(__powerpc__) || defined(__ppc__) || defined(__ppc64__) || \
     defined(__sparc__) || defined(__sparc) || defined(__s390__) || \
     defined(__SH4__) || defined(__alpha__) || \
     defined(_MIPS_ARCH_MIPS32R2)