Increase the size of the heap size available on sparc64 during

operation of "loader".  The dramatic increase in size of
SPA_MAXBLOCKSIZE in r304321 causes the heap space to be exhausted,
so malloc() fails, ultimately leading to a memcpy() with a
destination of 0x0.

MFC after:	3 days

stand/sparc64/loader/main.c

@@ -77,7 +77,7 @@ __FBSDID("$FreeBSD$");
 
 enum {
 	HEAPVA		= 0x800000,
-	HEAPSZ		= 0x1000000,
+	HEAPSZ		= 0x3000000,
 	LOADSZ		= 0x1000000	/* for kernel and modules */
 };