[ld, testsuite] Don't print to stdin for "readelf -w" to avoid buffer overflow

[Jiong Wang]

The following failures from ld-elf/compress.exp on arm-none-eabi are because all
of them are using "readelf -w" while "-w" output is really huge as it's
outputing all dwarf information (> 60000 lines for each of the following tests
on arm-none-eabi).  The output size seems has overflowed dejagnu internal buffer
in remote.exp/local_exec, that further test actions are stopped.

All these tests actually relies on file diff, so I think it's not necessary to
print the output on stdin.

This patch fixed this.

No regression on arm-none-eabi cross/native check-ld and x86 native check-ld, ld.log
for x86 reduced from ~40000 lines to ~26000.

OK for master?

UNRESOLVED: Link with zlib compressed debug output
UNRESOLVED: Link with zlib compressed debug output
FAIL: Link with zlib compressed debug output
UNRESOLVED: Link with zlib-gnu compressed debug output
FAIL: Link with zlib-gnu compressed debug output
UNRESOLVED: Link with zlib-gabi compressed debug output
FAIL: Link with zlib-gabi compressed debug output

ld/
2017-02-03  Jiong Wang  <jiong.wang@arm.com>

         * testsuite/ld-elf/compress.exp: Don't print to stdin for all
         "readelf -w".

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