Fix locking on PPC64

The compiler definition is capitalised on Linux.  Also, use a type with
the proper width and signedness.

--- jamvm-1.5.1/src/arch/powerpc.h.old	2007-09-16 21:32:27.000000000 -0500
+++ jamvm-1.5.1/src/arch/powerpc.h	2024-09-13 11:26:29.805601005 -0500
@@ -31,10 +31,10 @@
 /* Needed for i386 -- empty here */
 #define FPU_HACK
 
-#ifdef __ppc64__
+#ifdef __PPC64__
 #define COMPARE_AND_SWAP(addr, old_val, new_val)    \
 ({                                                  \
-    int result, read_val;                           \
+    uintptr_t result, read_val;                     \
     __asm__ __volatile__ ("                         \
                 li %0,0\n                           \
         1:      ldarx %1,0,%2\n                     \