xen/perf: Define .glob for the different hypercalls.

This allows us in perf to have this:

 99.67%  [kernel]             [k] xen_hypercall_sched_op
  0.11%  [kernel]             [k] xen_hypercall_xen_version

instead of the borring ever-encompassing:

 99.13%  [kernel]              [k] hypercall_page

[v2: Use a macro to define the name and skip]
[v3: Use balign per Jan's suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>