Support ECOFF COFF extension

[XVilka]

ECOFF is an extension of COFF developed for the MIPS platform, and was used by DEC Ultrix and Tru64 (previously Digital Unix and OSF/1), SGI Irix, Linux/MIPS and the Net Yaroze.

Also needed for #4007

See:

• https://github.com/ChrisRx/mipsel-ecoff-toolchain
• ecoff-bigmips not recognized by COFF-parser. NationalSecurityAgency/ghidra#356
• https://github.com/nihilus/snowman-tests/blob/master/mipsbin/fsn.ECOFF