kbuild: revert "save ARCH & CROSS_COMPILE ..."

Revert commit 575543347b5baed0ca927cb90ba8807396fe9cc9

It caused following issues:

- On architectures where ARCH= setting is used to select between
  32 and 64 bit this was no longer possible without "make mrproper"
- If ARCH was changed then kbuild refused to run "make mrproper"
  because ARCH had changed
- When CROSS_COMPILE was changed people were asked to run "make mrproper"
  but kbuild refused to run "make mrproper" because CROSS_COMPILE changed.
- Spaces in CROSS_COMPILE was not 'supported'
- If an non-existing ARCH= was used kbuild could get stuck

Lessons learned:
. Despite being simple and straghtforward people uses very different
  approaches when building the kernel.

. CROSS_COMPILE is sometimes used for ccache despite cache being
  only a CC frontend so one would have expected CC to be
  used for this purpose.

. And obviously this was not tested widely enough.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

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