external-toolchain: fix support

The recent commit adding the external toolchain wrapper has broken the
support for external toolchain. The check_arm_eabi, check_cplusplus
and check_cross_compiler_exists functions were using TARGET_CC, which
points to the toolchain wrapper, but at the moment those functions are
called, the wrapper hasn't been generated yet.

We fix this by passing to these functions the path to the C or C++
compiler they should use for their tests.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>