x86/kernel: increase kcov coverage under arch/x86/kernel folder

Currently kcov instrument is disabled for object files under
arch/x86/kernel folder.

For object files under arch/x86/kernel, actually just disabling the kcov
instrument of files:"head32.o or head64.o and sev.o" could achieve
successful booting and provide kcov coverage for object files that do not
disable kcov instrument.  The additional kcov coverage collected from
arch/x86/kernel folder helps kernel fuzzing efforts to find bugs.

Link to related improvement discussion is below:
https://groups.google.com/g/syzkaller/c/Dsl-RYGCqs8/m/x-tfpTyFBAAJ Related
ticket is as follow: https://bugzilla.kernel.org/show_bug.cgi?id=198443

Link: https://lkml.kernel.org/r/06c0bb7b5f61e5884bf31180e8c122648c752010.1690771380.git.pengfei.xu@intel.com
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Pengfei Xu <pengfei.xu@intel.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: <heng.su@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Kees Cook <keescook@google.com>,
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/kernel/Makefile

@@ -33,11 +33,10 @@ KCSAN_SANITIZE := n
 KMSAN_SANITIZE_head$(BITS).o				:= n
 KMSAN_SANITIZE_nmi.o					:= n
 
-# If instrumentation of this dir is enabled, boot hangs during first second.
-# Probably could be more selective here, but note that files related to irqs,
-# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
-# non-deterministic coverage.
-KCOV_INSTRUMENT		:= n
+# If instrumentation of the following files is enabled, boot hangs during
+# first second.
+KCOV_INSTRUMENT_head$(BITS).o				:= n
+KCOV_INSTRUMENT_sev.o					:= n
 
 CFLAGS_irq.o := -I $(srctree)/$(src)/../include/asm/trace