x86: Allow generating user-space headers without a compiler

When bootstrapping an architecture, it's usual to generate the kernel's
user-space headers (make headers_install) before building a compiler.  Move
the compiler check (for asm goto support) to the archprepare target so that
it is only done when building code for the target.

Fixes: e501ce95

 ("x86: Force asm-goto")
Reported-by: Helmut Grohne <helmutg@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180829194317.GA4765@decadent.org.uk