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: Ingo Molnar <mingo@elte.hu>