gcc: set the default target arch

gcc: set the default target arch

The default x86-64 architecture for target gcc (ie, the one in poky
build appliances) is native. Since we have a variety of build systems
it will occasionally produce instructions that don't work on all of
our development system.

Instead, set gcc's default architecture to the one specified in
TUNE_CC_ARCH, that guarantees that gcc-runtime and any binaries
produced are compatible with the target machine type.

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>