hexagon: work around compiler crash

Clang cannot translate the string "r30" into a valid register yet.

Link: https://github.com/ClangBuiltLinux/linux/issues/755
Link: http://lkml.kernel.org/r/20191028155722.23419-1-ndesaulniers@google.com
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Suggested-by: Sid Manning <sidneym@quicinc.com>
Reviewed-by: Brian Cain <bcain@codeaurora.org>
Cc: Allison Randal <allison@lohutok.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Richard Fontana <rfontana@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/hexagon/kernel/stacktrace.c

@@ -11,8 +11,6 @@
 #include <linux/thread_info.h>
 #include <linux/module.h>
 
-register unsigned long current_frame_pointer asm("r30");
-
 struct stackframe {
 	unsigned long fp;
 	unsigned long rets;
@@ -30,7 +28,7 @@ void save_stack_trace(struct stack_trace *trace)
 
 	low = (unsigned long)task_stack_page(current);
 	high = low + THREAD_SIZE;
-	fp = current_frame_pointer;
+	fp = (unsigned long)__builtin_frame_address(0);
 
 	while (fp >= low && fp <= (high - sizeof(*frame))) {
 		frame = (struct stackframe *)fp;