gdb: Remove stabs support from XCOFF inferiors

gdb: Remove stabs support from XCOFF inferiors

This commit is the second to last in the series fully removing support
for stabs in GDB, removing it from XCOFF inferiors. According to IBM's
AIX documentation[1], xcoff binaries can only have stabs or DWARF debug
info, meaning removing stabs seems pretty trivial, as anything that
isn't related to setting base information on the objfile or reading
dwarf can be removed.

WIP missing testing and double checking macros.

[1] https://www.ibm.com/docs/en/aix/7.3?topic=formats-xcoff-object-file-format