Linking failure on aarch64

[FX]

> Could you please try again with the attached patch, doing just a plain
> .../configure && make && make check?

I build with: ./configure --build=aarch64-apple-darwin20.0.0
otherwise config.guess returns arm-apple-darwin20.0.0 and it tries to build for 32-bit.

With your patch, and the above configure line, configuration succeeds but 8 tests fail with:

dyld: Symbol not found: __gmpn_invert_limb_table

which is used in bdiv_q_1.o but not available anywhere.

FX