MIPS: Fix `readelf -S bintest' test for n64 targets

MIPS: Fix `readelf -S bintest' test for n64 targets

Add a 64-bit traditional MIPS dump variant for the `readelf -S bintest'
test from binutils-all/readelf.exp, using a filename suffix according to
the rules set there, removing:

FAIL: readelf -S bintest

regressions with `mips64-linux-gnuabi64', `mips64el-linux-gnuabi64',
`mips64-openbsd', and `mips64el-openbsd' targets, which default to the
n64 ABI and consequently produce a section layout that is different from
what the generic dump pattern covers.

Co-Authored-By: Maciej W. Rozycki <macro@orcam.me.uk>
	binutils/
	* testsuite/binutils-all/readelf.s-64-tmips: New test variant.