[bios] Use an 8kB stack for x86_64

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/core/stack.S

 
 	.arch i386
 
+#ifdef __x86_64__
+#define STACK_SIZE 8192
+#else
+#define STACK_SIZE 4096
+#endif
+
 /****************************************************************************
  * Internal stack
  ****************************************************************************
 	.align 8
 	.globl _stack
 _stack:
-	.space 4096
+	.space STACK_SIZE
 	.globl _estack
 _estack: