compiler.h: add host_build()

compiler.h: add host_build()

Add a host_build() function, so that it's possible to
check for software being build with USE_HOSTCC without
relying on preprocessor conditions. In other words

 #ifdef USE_HOSTCC
 	host_only_code();
 #endif

can be written like this instead:

 if (host_build())
 	host_only_code();

This improves code readability and test coverage and
compiler will eleminate this unreachable code.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>