sys/fmt: Add print_s64_dec to header

[MrKevinWeiss]

Contribution description

It seems that print_s64_dec is implemented but not exposed in the header.
This PR exposes it and improves coverage of the fmt module.

Testing procedure

make all test -C tests/fmt_print/

CI should pass too.

Issues/PRs references

Split from #15950

[maribu]

ACK. Thanks also for extending the test properly.

[maribu]

Test output using nucleo-f767zi:

READY
s
START
main(): This is RIOT! (Version: 2021.04-devel-525-g73cd4-kevin)
If you can read this:
4294967295
-2147483648
FA
-2147483648
12345678
123456789ABCDEF0
18446744073709551615
-9223372036854775808
1.23450
Test successful.

[maribu]

This should make the CI happy :-)

diff --git a/tests/periph_ptp_clock/main.c b/tests/periph_ptp_clock/main.c
index f3afc03b73..22f82464d2 100644
--- a/tests/periph_ptp_clock/main.c
+++ b/tests/periph_ptp_clock/main.c
@@ -46,16 +46,6 @@
 static mutex_t sync_mutex = MUTEX_INIT_LOCKED;
 static atomic_uint_least64_t timestamp;
 
-static inline void print_s64_dec(int64_t _num)
-{
-    uint64_t num = _num;
-    if (_num < 0) {
-        print_str("-");
-        num = -_num;
-    }
-    print_u64_dec(num);
-}
-
 static void speed_adj_cb(void *arg, int chan)
 {
     (void)arg;

[maribu]

Re-ACK

[MrKevinWeiss]

Thanks for the review!