Add support to detect arm vs armv6

There are two different versions of the ARM ABI depending on the
TARGET_ARCH. As these are sligntly different a package built for
one may not work on another. We need to detect which one we are on
by parsing the .ARM.attributes section.

This will only work on the ARM EABI as this section is part of the
ABI definition. As armv6 only supports the ARM EABI this is not a
problem for the oabi.

Older versions of libelf in FreeBSD fail to read the
.ARM.attributes section needed. As armv6 is unsupported on these
versions we can assume we are running on arm.

Submitted by: andrew
Approved by: re (delphij)
Obtained from: pkgng git