Lunch: don't use a toolchain that doesn't exist.

"lunch sdk-eng" on cygwin tries to use a gcc.exe from the
prebuild arm toolchain, but it doesn't exist. This prevents
from setting variables to point on a gcc toolchain if there
is none for the given arch.

Change-Id: I3b72220663687f9fdaacc050899aef00632f2c3c
3 files changed