x86/build: Fix compiler support check for CONFIG_RETPOLINE

x86/build: Fix compiler support check for CONFIG_RETPOLINE

It is troublesome to add a diagnostic like this to the Makefile
parse stage because the top-level Makefile could be parsed with
a stale include/config/auto.conf.

Once you are hit by the error about non-retpoline compiler, the
compilation still breaks even after disabling CONFIG_RETPOLINE.

The easiest fix is to move this check to the "archprepare" like
this commit did:

  829fe4aa9ac1 ("x86: Allow generating user-space headers without a compiler")

Reported-by: Meelis Roos <[email protected]>
Tested-by: Meelis Roos <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Acked-by: Zhenzhong Duan <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Zhenzhong Duan <[email protected]>
Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
Link: http://lkml.kernel.org/r/[email protected]
Link: https://lkml.org/lkml/2018/12/4/206
Signed-off-by: Ingo Molnar <[email protected]>