Configure avr-gcc with `--with-gnu-ld`

[sprintersb]

Configuring GCC with --with-gnu-ld can improve the handling of @-files:

Only when the driver knows that the GNU linker is used, it will put options / objects for linking in @-file. This may be required on Windows when the inflated command line is too long for Windows. The behaviour of collect2 also depends on that flag: When the driver calls collect2 for linking with an @-file, collect2 will call ld with an @-file only with the GNU linker.

The configure scripts can't always work this out, e.g. for canadian-cross builds, because configure cannot call the host's ld.exe.