x86: Fix RETPOLINE_CFLAGS check

The 4.9.11 merge did not properly resolve the conflict between commit
084a9b8f ("Revert "x86: Allow generating user-space headers without
a compiler"") in kernel/common and commit d92c66b3 ("x86/build: Fix
compiler support check for CONFIG_RETPOLINE"). The archprepare target
should have been added back, otherwise this check will not work
properly. This can be verified by enabling CONFIG_RETPOLINE and
inverting the RETPOLINE_CFLAGS check; before this patch, the build will
succeed but after, the build will properly terminate.

Change-Id: I18b13ce1e73c2023e2e1ce07593744b3f7929ac9
Fixes: a87fb6b9 ("Merge 4.19.11 into android-4.19")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>