toolchain-wrapper: fix breakage after host/usr removal

The toolchain wrapper, when called through PATH, strips the last three
levels of /proc/self/exe to find HOST_DIR. However, after the host/usr
removal, this should be just two levels.

The toolchain wrapper has different logic for when it is called with a
full path (i.e. $HOST_DIR/usr/bin/arm-linux-gcc) then when it is called
through the PATH (i.e. just arm-linux-gcc). The latter is never used
internally in Buildroot, that's why this wasn't discovered through
testing.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Mark Jackson <mpfj-list@newflow.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>