Remove all the handcrafted assembly in hwpmc_armv7.c and use the
common (autogenerated) versions. Removes extra vertical space,
and makes it easier to grep for usage throughout the tree.
Conditionally compile only for arm6 [1] (yes sounds odd but is right).

Submitted by: andrew [1]
Reviewed by: gnn, andrew (ian earlier version I think)
Differential Revision: https://reviews.freebsd.org/D2159
Obtained from: Cambridge/L41
Sponsored by: DARPA, AFRL