sparc: move stack ouf of the way

This was copied from PPC and is not done on any other architecture.

Having the stack use the kernel address results in the kernel being
loaded elsewhere. Not really a problem (the kernel is relocatable), but
it's annoying when debugging as addresses need to be converted between
the elf file and the actual loading address.

Change-Id: Iab25af1fb8998b7ef5c583459c134aea80be8d72
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3581
Reviewed-by: waddlesplash <[email protected]>

src/system/boot/platform/openfirmware/arch/sparc/cpu.cpp

@@ -89,7 +89,7 @@ boot_arch_cpu_init(void)
 
 	// allocate the kernel stacks (the memory stuff is already initialized
 	// at this point)
-	addr_t stack = (addr_t)arch_mmu_allocate((void*)0x80000000,
+	addr_t stack = (addr_t)arch_mmu_allocate(NULL,
 		cpuCount * (KERNEL_STACK_SIZE + KERNEL_STACK_GUARD_PAGES * B_PAGE_SIZE),
 		B_READ_AREA | B_WRITE_AREA, false);
 	if (!stack) {