make all library-internal static buffers thread-local

We have a number of library-internal static global buffers which are
mainly used for various stringification functions.  This worked as
all of the related Osmocom programs were strictly single-threaded.

Let's make those buffers at least thread-local.  This way every thread
gets their own set of buffers, and it's safe for multiple threads to
execute the same functions once.  They're of course still not
re-entrant.  If you need re-entrancy, you will need to use the _c()
or _buf() suffix version of those functions and work with your own
(stack or heap) buffers.

Change-Id: I50eb2436a7c1261d79a9d2955584dce92780ca07
diff --git a/src/sim/core.c b/src/sim/core.c
index 63b3000..b93633c 100644
--- a/src/sim/core.c
+++ b/src/sim/core.c
@@ -300,8 +300,7 @@
 
 char *osim_print_sw(const struct osim_card_hdl *ch, uint16_t sw_in)
 {
-	/* FIXME: do we want to mark this as __thread? */
-	static char sw_print_buf[256];
+	static __thread char sw_print_buf[256];
 	return osim_print_sw_buf(sw_print_buf, sizeof(sw_print_buf), ch, sw_in);
 }