utils: Use fprintf to print the assertion and generate a backtrace

This is changing the semantic of the assert. The regression tests
now either need to check the stderr result, the exit status or print
a message when all tests are completed.

This is not that bad as the osmo_generate_backtrace is printing to
the stdout right now.

include/osmocom/core/utils.h

@@ -53,7 +53,8 @@ do {								\
 
 #define OSMO_ASSERT(exp)    \
 	if (!(exp)) { \
-		printf("Assert failed %s %s:%d\n", #exp, __FILE__, __LINE__); \
+		fprintf(stderr, "Assert failed %s %s:%d\n", #exp, __FILE__, __LINE__); \
+		osmo_generate_backtrace(); \
 		abort(); \
 	}