Bug#950645: src:gmp, add lib64 for i386 x32 mipsel etc

[YunQiang Su]

Package: src:gmp
Version: 6.2.0+dfsg-3

Since binutils/gcc meet some problem due to 2GB/3GB memory limitation
on 32bit system.
Now one solution is to have host64-target32 toolchains if the arch
have compatible 64bit arch,
just like x86, mips, powerpc.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=950527


-- 
YunQiang Su