Fix Mesa build with clang 10 on mips64.

On mips64, the compiler does not allow use of non-zero argument with
__builtin_frame_address(). However, the returned frame address is only
used when PIPE_ARCH_X86 is defined. The compile error can be avoided
by making #ifdef PIPE_ARCH_X86 cover the getting of frame address too.

The argument checking of __builtin_frame_address() has been present
as a debug assert in clang 8. In clang 10, there is a proper runtime
check for the argument. This is why the build has not failed before.

OK jsg@

lib/mesa/src/gallium/auxiliary/util/u_debug_stack.c

@@ -265,6 +265,7 @@ debug_backtrace_capture(struct debug_stack_frame *backtrace,
    }
 #endif
 
+#ifdef PIPE_ARCH_X86
 #if defined(PIPE_CC_GCC) && (PIPE_CC_GCC_VERSION > 404) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wframe-address"
@@ -279,7 +280,6 @@ debug_backtrace_capture(struct debug_stack_frame *backtrace,
    frame_pointer = NULL;
 #endif
 
-#ifdef PIPE_ARCH_X86
    while (nr_frames) {
       const void **next_frame_pointer;