Disable cross-gdb in nios2-elf-mingw32.

Building cross-gdb in canadian cross requires expat/ncurses for the
host. Currently, 300-gdb.sh only builds expat/ncurses for the target
(for native-gdb). For cross-gdb on regular cross (build==host), expat
and ncurses are expected to be provided by the host.

There are two approaches possible:

- If building for canadian cross, build expat/ncurses for cross-gdb
  just as the native-gdb does.
- Promote expat/ncurses to first class citizens and build them as
  companion libs during the build of the build-to-host toolchain.

I am leaning towards the latter approach - it would also allow to
specify the versions for expat/ncurses rather than have them hardcoded
in 300-gdb.sh - but would appreciate feedback.

Signed-off-by: Alexey Neyman <stilor@att.net>

samples/nios2-elf-mingw32/crosstool.config

@@ -13,7 +13,7 @@ CT_HOST="i686-w64-mingw32"
 CT_BINUTILS_V_2_25=y
 CT_CC_LANG_CXX=y
 CT_DEBUG_gdb=y
-CT_GDB_V_7_8_1=y
+# CT_GDB_CROSS is not set
 CT_MPFR_V_3_1_2=y
 CT_ISL_V_0_12_2=y
 CT_MPC_V_1_0_2=y