BUG: Cross compilation results in wrong executable name

[fzwoch]

Currently Makefile assumes it is doing a host compile always:

    ifndef CPU
        CPU := $(shell uname -m | sed -e s/i.86/i386/ -e s/amd64/x86_64/ -e s/sun4u/sparc64/ -e s/arm.*/arm/ -e s/sa110/arm/ -e s/alpha/axp/)
    endif
    ifndef SYS
        SYS := $(shell uname -s)
    endif

So cross compile will result in the wrong info for these two variables. Can be observed in current CI on macOS where the arm64 target is being build on an intel Mac:

Run .github/workflows/scripts/homebrew.sh build-arm64
[..]
[LD] ezquake-darwin-x86_64