gdb/mdebug: Remove stabs support from mips inferiors

gdb/mdebug: Remove stabs support from mips inferiors

Ostensibly, the mdebugread.c is about reading debug information in the
ecoff format, but it also supports stabs-in-ecoff according to comments
in there, and also relied in some stabs facilities for ecoff reading
itself. This commit takes the first step in removing stabs support by
removing those dependencies from mdebug. And in order to support
stabs-in-ecoff, mipsread would also call stabsread_new_init.

Removing stabs-in-ecoff is trivial, as the code was well demarcated with
comments mentioning where stabs was read. Plus the call to
stabsread_new_init in mipsread can be trivially removed.

Another simple removal was the dependence on stabs_end_psymtabs: because
the local variables dependencies_used and includes_used were only touched
by stabs-reading code, they are always 0 in the new version, which means
we can find the exact code path that'd be followed in stabs_end_psymtab
and move the relevant lines to mdebug instead.

After all those, the last remaining dependency is when reading a fortran
common block from an inferior compiled by SGI fortran compilers (and
maybe more). The block could have no address, meaning it'd need to be
fixed after all the minimal symbols have been read. This was done by
adding the symbol to the stabs global_sym_chain, then calling
scan_file_globals to fix them up. This commit copies all the necessary
code for handling the fortran symbols onto mdebug, technically making
some code duplication, but since stabsread will be removed soon, this
shouldn't be too concerning.

This change was tested in the compile farm's mips64 machine (number
230), where it actually seems to have solved some 50 failures in the
testsuite, not including changes in tests from gdb.threads, as those are
often very racy. I'm not sure if these were true fixes or racy cases,
but since the new version has no newly introduced fails, only fewer of
them, I'm inclined to think this change is at least harmless.