ld: testsuite: Simplify emulation check in libgot tests

The x86 libgot-1 tests are the only ones in all testsuites that use a
new ld -V -m<emul under test> idiom to check whether to run the tests.

Rather than open-coding the check everywhere while relying on that
idiom, this patch introduces a new proc to directly check the ld -V
output for the emulation in question.

Tested on {x86_64,i686}-pc-linux-gnu and {amd64,i386}-pc-solaris2.11.

2026-01-24  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	ld:
	* testsuite/lib/ld-lib.exp (ld_supports_emul): New proc.
	* testsuite/ld-i386/binutils.exp: Use it.
	* testsuite/ld-x86-64/binutils.exp: Likewise.