mingw: use a more canonical method to fix the CPU reporting

In `git version --build-options`, we report also the CPU, but in Git for Windows we actually cross-compile the 32-bit version in a 64-bit Git for Windows, so we cannot rely on the auto-detected value. In 3815f64b0dd9 (mingw: fix CPU reporting in `git version --build-options`, 2019-02-07), we fixed this by a Windows-only workaround, making use of magic pre-processor constants, which works in GCC, but most likely not all C compilers. As pointed out by Eric Sunshine, there is a better way, anyway: to set the Makefile variable HOST_CPU explicitly for cross-compiled Git. So let's do that! This reverts commit 3815f64b0dd983bdbf9242a0547706d5d81cb3e6 partially. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>