x86: debugctlmsr kconfig

This adds the (internal) Kconfig macro CONFIG_X86_DEBUGCTLMSR,
to be defined when configuring to support only hardware that
definitely supports MSR_IA32_DEBUGCTLMSR with the BTF flag.

The Intel documentation says "P6 family" and later processors all have it.
I think the Kconfig dependencies are right to have it set for those and
unset for others (i.e., when 586 and earlier are supported).
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/Kconfig.cpu

@@ -399,3 +399,7 @@ config X86_MINIMUM_CPU_FAMILY
 	default "4" if X86_32 && (X86_XADD || X86_CMPXCHG || X86_BSWAP || X86_WP_WORKS_OK)
 	default "3"
 
+config X86_DEBUGCTLMSR
+	bool
+	depends on !(M586MMX || M586TSC || M586 || M486 || M386)
+	default y