Fix ld action in run_dump_test

Fix ld action in run_dump_test

run_dump_test proposes an ld action but when trying to make use of it in
a gas test it gave me some Tcl error. It turns out that it references
the check_shared_lib_support procedure and ld_elf_shared_opt variable
both only available in ld-lib.exp. I've thus moved the procedure in
binutils-common.exp and defined the variable needed in the various
default.exp of testsuite that seem to be using run_dump_test.

Since check_shared_lib_support itself references the ld variable not
defined in binutils-common I've defined it from LD in run_dump_test and
fixed LD and LDFLAGS to be defined as expected by run_dump_test in the
various default.exp of testsuite using run_dump_test.

2018-11-01  Thomas Preud'homme  <thomas.preudhomme@linaro.org>

binutils/
	* testsuite/config/default.exp: Define LD, LDFLAGS and
	ld_elf_shared_opt.
	* testsuite/lib/binutils-common.exp (check_shared_lib_support): Moved
	from ld-lib.exp.
	(run_dump_test): Set ld to $LD.

gas/
	* testsuite/config/default.exp: Define LD, LDFLAGS and
	ld_elf_shared_opt.

ld/
	* testsuite/lib/ld-lib.exp (check_shared_lib_support): Moved to
	binutils-common.exp.