fix(rtp): avoid false-positive overflow detected by UBSan with timestamp macros

See merge request !1

(cherry picked from commit e06fccea)

add more precise check on std atomic, specialy to avoid using uncompatible implementatiwqon (gcc 4.9 android)

add msgown(), a function to know whether the underlying buffer is shared or not. Not used by anyone at this time.

Add ortp_gettimeofday() fallback in case the RtpTransport doesn't set the timestamp onto the mblk_t. This is the case with Tunnel mode in liblinphone.