Paul and I got trapped a few times by not seeing the effects of applying
a patch to the nolibc source code until a "make clean" was issued in
the nolibc directory. It's particularly annoying when trying to confirm
that a proposed patch really solves a problem (or that reverting it
reintroduces the problem).

The reason for the sysroot not being rebuilt was that it can be quite
slow. But in fact it's only slow after a "make clean" issued at the
kernel's topdir, because it's the main "make headers" that can take a
tens of seconds; as long as "usr/include" still contains headers, the
"headers_install" phase is only a quick "rsync", and rebuilding the
whole nolibc sysroot takes a bit less than one second, which is perfectly
acceptable for a test, even more once the time lost caused by misleading
results is factored in.

This patch marks the sysroot target as phony and starts by clearing
the previous sysroot for the current architecture before reinstalling
it. Thanks to this, applying a patch to nolibc makes the effect
immediately visible to "make nolibc-test":

  $ time make -j -C tools/testing/selftests/nolibc nolibc-test
  make: Entering directory '/k/tools/testing/selftests/nolibc'
    MKDIR   sysroot/x86/include
  make[1]: Entering directory '/k/tools/include/nolibc'
  make[2]: Entering directory '/k'
  make[2]: Leaving directory '/k'
  make[2]: Entering directory '/k'
    INSTALL /k/tools/testing/selftests/nolibc/sysroot/sysroot/include
  make[2]: Leaving directory '/k'
  make[1]: Leaving directory '/k/tools/include/nolibc'
    CC      nolibc-test
  make: Leaving directory '/k/tools/testing/selftests/nolibc'

  real    0m0.869s
  user    0m0.716s
  sys     0m0.149s

Cc: "Paul E. McKenney" <paulmck@kernel.org>
Link: https://lore.kernel.org/all/20221021155645.GK5600@paulmck-ThinkPad-P17-Gen-1/Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>