[gmp] No asm for mingw-dynamic

[dg0yt]

When enabling assembler (incl. runtime selection via feature "fat"), only functions implemented in C appear in the import lib for mingw dynamic. This breaks downstream ports such as nettle.

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[Neumann-A]

Hmm is this an issue for static builds or only for dynamic builds? Maybe the default visibility needs to be switched?

[Neumann-A]

hmm gmp[core,fat]:x64-windows doesn't build for me. I get a bunch of E:\vcpkg_folders\master_clean\buildtrees\gmp\src\v6.2.1-0c723d4b6f.clean\gmp-impl.h(4657): error C2071: 'add_n': illegal storage class

[dg0yt]

I can't say much about windows triplets (clang-cl). This PR is for mingw (gcc).
However, I can confirm that mpn_add_n is one of the missing functions with asm for x64-mingw-dynamic.
Providing a list of exported symbols may be a better fix, but I can't handle this ATM.