Update comments on stepping over resolver code.

Update comments on stepping over resolver code.

This updates the comments on the step-over-resolver mechanism a bit,
adjusting it to refer to the gdbarch hooks instead of the old macros;
to mention the in_dynsym_resolve_code hook of the target_so_ops
vector; and to American English spelling (signalling->signaling).

gdb/
2013-06-26  Pedro Alves  <palves@redhat.com>

	* infrun.c: Update comments on stepping over runtime loader
	dynamic symbol resolution code.