PellesC: Add support for ASM_POASM language assembler

The Pelles C compiler does not support ASM, and the `poasm` assembler
uses a distinct syntax.  Add a dedicated language as we do for similar
cases from other vendors.  Leave documentation commented out for now
because the PellesC toolchain support in general is not yet documented.

Fixes: #27758
Issue: #21536
Co-authored-by: Serguei E. Leontiev <leo@sai.msu.ru>