arch: add support for "jaguar" AMD CPU optimisations

AMD Jaguar ( https://en.wikipedia.org/wiki/Jaguar_%28microarchitecture%29

 ) is
suddenly a popular architecture since it is used in the PS4 and the XBox One.
Many embedded systems are also likely to use it in the next years.

This patch adds support for GCC architecture-specific optimisations and
tuning for these CPUs.
These optimizations are available with GCC 4.8+.

Signed-off-by: Adrien Beraud <adrien.beraud@savoirfairelinux.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>