goarch.bbclass: Define HOST_GO386 and TARGET_GO386

goarch.bbclass: Define HOST_GO386 and TARGET_GO386

These variables are used to control the floating-point instructions emitted by
the go compiler for x86 architectures.

The default is '387' which disables the use of sse/sse2 instructions and is safe
to use on all x86 processors from the i486 onwards.

If TUNE_FEATURES contains a feature set which is known to support sse/sse2
instructions then they are set to 'sse2' to enable the use of these
instructions. This is suitable for most processors from Pentium 4 onwards.

Only the 'core2' and 'corei7' TUNE_FEATURES are defined in the oe-core layer and
are known to support ss2 instructions. Other layers may introduce additional
tunings which support sse2 instructions and for these layers, TARGET_GO386
or go_map_386 should be overridden as needed.

Signed-off-by: Paul Barker <pbarker@toganlabs.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>