gmp: Add --build=core2-linux-gnu [Linux]
gmp: Add --build=core2-linux-gnu [Linux]

Add --build=core2-linux-gnu when building a bottle for Linuxbrew.
Otherwise GMP uses the popcnt instruction, which is not
available on all supported CPUs.

Fixes https://github.com/Linuxbrew/homebrew-core/issues/1683

LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,LinuxbrewTestBot/homebrew-core,Linuxbrew/homebrew-core,rwhogg/homebrew-core