Use coffread_objfile throughout coffread.c

coffread.c:coff_symtab_read sets a global objfile while working.  It
seemed nicer to me to hoist this so that it is set by the main entry
point, and then used throughout the file.

Normally I wouldn't encourage the use of globals, but shortly we'll be
replacing these with an object.

Note that while coff_symtab_read cleared coffread_objfile when it was
finished, I haven't preserved this in the new patch.  First, this
should have been done with a scoped_restore.  Second, this is the only
global to get this treatment.  Third, it's not necessary at all.  And,
finally, this will be moot anyway when the globals are removed.

Approved-By: Simon Marchi <simon.marchi@efficios.com>