cm3: scb: C11 keyword compliance

Avoid use of the gnu specific "asm" keyword, and use the __asm__
keyword, as used everywhere else in the library.  This fixes compilation
in C11, and unifies all uses of asm literals in the codebase.

Reported-by: @dragonmux
Fixes: https://github.com/libopencm3/libopencm3/pull/1425

Signed-off-by: Karl Palsson <karlp@tweak.net.au>

include/libopencm3/cm3/scb.h

@@ -549,7 +549,7 @@ struct scb_exception_stack_frame {
 
 #define SCB_GET_EXCEPTION_STACK_FRAME(f)				\
 	do {								\
-		asm volatile ("mov %[frameptr], sp"			\
+		__asm__ volatile ("mov %[frameptr], sp"			\
 			      : [frameptr]"=r" (f));			\
 	} while (0)