[committed] Use "nm" rather than "objdump -dr" on a 1GB MIPS testcase

[Adam Nemet]

Richard Sandiford <rdsandiford@googlemail.com> writes:
> Noticed while running the testsuite on an old laptop (512MB)
> that the gas testsuite was thrashing and taking a long time to run.
> It turned out that we were using "objdump -dr" on a 1GB file to
> verify symbol values.

I used to sometimes manually comment out this test if I needed to run
the tests repeatedly on older machines.  Thanks for fixing this!

Adam