Test that gcc -B picks up new ld

Test that gcc -B picks up new ld

gcc configured using --with-ld always uses that particular version of
ld if it exists.  This patch adds a check that the ld version reported
directly from ld-new matches that reported via gcc.  I chose to
compare the ld --version output rather than a more strict test (for
example by gcc --print-prog-name=ld), because the version test allows
a user to run the ld testsuite after installing a new ld (in the place
expected by gcc).

Sample output:

Running target unix
Using /usr/share/dejagnu/baseboards/unix.exp as board description file for target.
Using /usr/share/dejagnu/config/unix.exp as generic interface file for target.
Using /home/alan/src/binutils-gdb/ld/testsuite/config/default.exp as tool-and-target-specific interface file.
ERROR: ************************************************************************
ERROR: Your compiler driver ignores -B when choosing ld.
ERROR: You will not be testing the new ld in many of the following tests.
ERROR: It seems you will be testing /usr/bin/x86_64-w64-mingw32-ld instead.
ERROR: ************************************************************************
WARNING: Assuming target board is the local machine (which is probably wrong).
You may need to set your DEJAGNU environment variable.
Running /home/alan/src/binutils-gdb/ld/testsuite/ld-aarch64/aarch64-elf.exp ...

	* testsuite/lib/ld-lib.exp (run_host_cmd): Check that gcc -B
	works.