external-toolchain: adjust tests on TARGET_CC and TARGET_CXX

Following the changes to TARGET_CC/TARGET_CXX to include the --sysroot
option, these variables not only contain the path to the compiler, but
also the --sysroot option. For that reason, we cannot anymore just use
"test -x" to test for the compiler presence. Instead, we see if
$(TARGET_CC) -v and $(TARGET_CXX) -v return a zero status.

Moreover, --sysroot now needs to be filtered out of $(TARGET_CC) and
not $(TARGET_CFLAGS) when asking the toolchain for its original
sysroot and arch sysroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>