oe/elf: don't regenerate machine data on every call

oe/elf: don't regenerate machine data on every call

Every time oe.elf.machine_dict() is called a large dictionary is created
and returned. However, the "arch" package_qa test will call this method
for every binary in a package, which results in a lot of dictionary
creation.

Concrete exmaple: in running ltp:do_package_qa, the arch test takes 25%
of the runtime, and opitimising the machine_dict() call to cache the
generated dictionary reduces the runtime from 57s to 44s.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>