Create libelffile library for ELF file manipulation.

Move some of our tooling to library to make it reusable.

Remove MIPS support from the ELF builder.  This is slightly
easier than making it independent of the runtime.

Bug: 110133331
Test: test.py -b --host
Change-Id: I93343808d0e27ee8e1117e713a2503e8179fc245