gdb: also force ELF support for gdbhost

Similar to how we do for target (ee39d53ce3ee (Fix GDB BFD test linking)).

Gdb comes with an embedded copy of libiberty, but binutils also installs
libiberty.a into HOST_DIR. The gdb configure script tries to link against
this one rather than the gdb version when it checks for ELF support.
This may fail if those versions are not compatible, leading to obscure
error messages from gdb at runtime such as:

I'm sorry, Dave, I can't do that. Symbol format `elf32-$ARCH' unknown.

Fix it by forcing ELF support.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>