x86: drop largely defunct gas emulations

x86: drop largely defunct gas emulations

Both ELF and COFF have various sub-flavors, each of which would then
require its own emulation: Right now when configuring a COFF/PE
secondary target (with perhaps an ELF primary one), one gets plain COFF
emulation rather than COFF/PE one.

As such a multitude of emulations would be unwieldy (and likely fragile)
drop gas emulations altogether instead.