Revert "x86: disable X86_PTRACE_BTS"

This reverts commit 40f15ad8.

The CONFIG_X86_PTRACE_BTS bugs have been fixed via:

 c5dee617: x86, bts: memory accounting
 bf53de90: x86, bts: add fork and exit handling
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/Kconfig.cpu

@@ -521,7 +521,6 @@ config X86_PTRACE_BTS
 	bool "Branch Trace Store"
 	default y
 	depends on X86_DEBUGCTLMSR
-	depends on BROKEN
 	help
 	  This adds a ptrace interface to the hardware's branch trace store.