conf/sdk/x86-64: Set march specifically

This is really no change since it is currently the default in gcc but
making it explicit matches the other SDKMACHINE configs and means if gcc
ever changes, or we switch compilers, the flag is present. It also makes
it clear from the config which tuning is in use.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>