Add translation table from current pkg architecture identifiers to their
MACHINE_ARCH equivalents.

Note that armv6hf is missing as pkgng's type strings currently are not
capable of representing it (see comment about FreeBSD "not supporting"
this configuration in pkg_elf.c). Where pkg makes distinctions FreeBSD
does not (EABI vs. OABI for ARM), the list is arranged so that the best
option (EABI) appears first for reverse lookups.