gdb: get gdbarch from syscall_catchpoint instead of location

gdb: get gdbarch from syscall_catchpoint instead of location

I noticed some methods of syscall_catchpoint doing this:

  struct gdbarch *gdbarch = loc->owner->gdbarch;

`loc` is the list of locations of this catchpoint.  Logically, the owner
the locations are this catchpoint.  So this just ends up getting
this->gdbarch.  Remove the unnecessary indirection through the loc.

syscall_catchpoint::print_recreate does something slightly different,
getting its arch from the loc:

  struct gdbarch *gdbarch = loc->gdbarch;

I suppose it's always going to be the same arch, so get it from the
catchpoint there too.

Change-Id: I6f6a6f8e0cd7cfb754cecfb6249e71ec12ba4855
Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>