[doc] Remove references to mips-tfile on MIPS

[Joseph S. Myers]

Not strictly related to this patch, but there are other cleanups possible 
because of the only-used-on-Tru64 nature of mips-tdump/mips-tfile.  In 
particular, there are seven target macros (all undocumented) used by those 
programs and nowhere else in GCC: ALIGN_SYMTABLE_OFFSET CODE_MASK 
MIPS_IS_STAB MIPS_MARK_STAB MIPS_UNMARK_STAB SHASH_SIZE THASH_SIZE.  
Since those are only defined in alpha.h and only used on those programs, 
it would be better to hardcode the definitions inside 
mips-{tdump,tfile}.c, remove them from alpha.h and so eliminate seven 
target macros (about 1% of the total).  If you also hardcode the right 
definition of MIPS_DEBUGGING_INFO (used both in mips-tfile and elsewhere) 
then these programs should no longer depend on target macros and their 
tm.h includes can be removed.

(There are many other instances of #if conditionals in those programs, all 
of which are suspect since the programs are native-only for a single 
target and so shouldn't need conditional compilation at all.  But those 
other conditionals aren't relevant to target macro elimination.)

-- 
Joseph S. Myers
joseph@codesourcery.com