gdb: properly pass the PATH when building gdb for the host

When building gdb for the host, we properly pass the PATH (through
HOST_CONFIGURE_OPTS) during the configure step, but we forget to do so
for the compilation step.

The result of this is that when the Crosstool-NG backend is used, gawk
is built and installed in $(HOST_DIR), as a dependency of the
crosstool-ng package.

Then, the host gdb configure script detects this gawk binary
($(HOST_DIR) is in the PATH), and assumes gawk is
available. Unfortunately, during the compilation step, it fails to
find the expected gawk binary, because $(HOST_DIR) is no longer in the
PATH. This causes the following build failure:

  http://autobuild.buildroot.org/results/067d0c2ea01673ba98ec11de2426f1ab92dac800/build-end.log



In order to fix this, we simply call the compilation step of gdb for
the host with $(HOST_MAKE_ENV), as it should have been done from the
beginning.

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