maint: fix dependency of man/arch.1

maint: fix dependency of man/arch.1

The following is a reproducer for the wrong dependency:

  $ ./configure --enable-install-program=arch
  $ make
  $ rm -f src/arch man/arch.1
  $ make man/arch.1
    GEN      man/arch.1
  help2man: can't get `--help' info from man/arch.td/arch
  Try `--no-discard-stderr' if option outputs to stderr
  Makefile:14378: recipe for target 'man/arch.1' failed
  make: *** [man/arch.1] Error 127

* man/local.mk (man/arch.1): Change to depend on src/arch rather than
src/uname: while the arch binary depends on uname.c and uname-arch.c,
its man page depends on the arch binary.

Reported downstream by Rodrigues Goldwyn <rgoldwyn@suse.com> in
https://build.opensuse.org/request/show/415172