Tidy bfdio to consistenly use containing archive

[Maciej W. Rozycki]

Hi Alan,

> It's possible to have a valid "where" field for archives by always
> using and updating the containing archive BFD.  That's what this patch
> does.  Note that cache.c used to find the containing archive BFD
> anyway for the iostream, so we're not really doing extra work, just
> transferring it up to the correct abstraction level.
> 
> The patch also gets rid of some hacks.  bfd_tell was called when
> bfd_seek failed, in an attempt to correct "where".  That's got to be
> papering over another problem, so that code has been removed.
> bfd_read also had an "optimiziation" to return early when the number
> of bytes was zero, and bfd_seek optimized calls that didn't move the
> file pointer.  This was covering for a coff_slurp_line_table bug where
> IO was attempted on a pe-dll BFD without an iovec.

 It might be either the removal of what you call 'an "optimization"' or 
something else with your change, but in any case it has caused a serious 
performance regression seen in the GAS test suite with one of the MIPS 
test cases.  Execution times and the load before and after commit 
5c4ce239a3ab are respectively:

$ /usr/bin/time ../as-new   -o dump.o .../gas/testsuite/gas/mips/align.s
0.16user 0.31system 0:05.19elapsed 9%CPU (0avgtext+0avgdata 23904maxresident)k
0inputs+0outputs (0major+1590minor)pagefaults 0swaps

and:

$ /usr/bin/time ../as-new   -o dump.o .../gas/testsuite/gas/mips/align.s
2.48user 20.18system 0:33.97elapsed 66%CPU (0avgtext+0avgdata 23904maxresident)k
0inputs+0outputs (0major+1590minor)pagefaults 0swaps

This causes intermittent test suite failures with MIPS targets:

mips64-img-linux  +FAIL: MIPS align maximum
mips64-kfreebsd-gnu  +FAIL: MIPS align maximum
mips64-linux  +FAIL: MIPS align maximum
mips64-mti-linux  +FAIL: MIPS align maximum
mips64el-freebsd  +FAIL: MIPS align maximum
mips64el-kfreebsd-gnu  +FAIL: MIPS align maximum
mips64el-linux  +FAIL: MIPS align maximum
mips64el-openbsd  +FAIL: MIPS align maximum
mips64el-ps2-elf  +FAIL: MIPS align maximum
mips64vr-elf  +FAIL: MIPS align maximum

due to timeouts, which the regressed execution time in my test environment 
(where multiple test suites are run in parallel) is clearly on the verge 
of, e.g.:

../as-new   -o dump.o .../gas/testsuite/gas/mips/align.s
Executing on host: sh -c {../as-new   -o dump.o .../gas/testsuite/gas/mips/align.s 2>&1}  /dev/null gas.out (timeout = 300)
WARNING: program timed out.

.../gas/testsuite/../../binutils/nm-new  -n dump.o
Executing on host: sh -c {.../gas/testsuite/../../binutils/nm-new  -n dump.o >dump.out 2>gas.stderr}  /dev/null  (timeout = 300)
.../gas/testsuite/../../binutils/nm-new: dump.o: file format not recognized
FAIL: MIPS align maximum

 Can you please have a look into it?

  Maciej