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

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>