toolchain/gcc: make sure to pass $(HOST_DIR)/usr/bin in the PATH

Since gcc doesn't use the package infrastructure, it doesn't get all
the good generic environment variables, and forgets to get
$(HOST_DIR)/usr/bin in its PATH. This prevents gcc from finding and
using host tools built by Buildroot.

This patch therefore ensures that $(HOST_MAKE_ENV) or
$(TARGET_MAKE_ENV) are passed at the appropriate locations. It will be
useful for a later patch that makes gcc depend on host-flex/host-bison
in some situations.

Original patch by Thomas Petazzoni.

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