Revert "uname: switch machine to HW_MACHINE_ARCH"

Reverting because of issue in Makefile.inc1 during native builds:
make[1]: “.../freebsd/Makefile.inc1" line 163: Unknown target aarch64:aarch64.

Since I only tested this patch with make universe on amd64, this issue wasn't caught.

This reverts commit 83bf6ab568293e325f437342cdb87a626353e27c.