arm64: struct debug_info: Check CONFIG_HAVE_HW_BREAKPOINT

Check if CONFIG_HAVE_HW_BREAKPOINT is enabled before compiling in extra
data required for hardware breakpoints. Compiling out this code when hw
breakpoints are disabled saves about 272 bytes per struct task_struct.

Signed-off-by: Chris Redmon <credmonster@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>