api: Rework menu, and make it depend on CC_IS_GCC

We can only use the old U-Boot API for standalone applications when
building U-Boot with GCC as it relies upon the "gd is a register" trick
that only GCC supports. Further, rework the rest of the options so that
they are in the API menu and only visible if API support is enabled.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>