Provide dummy libraries for alpha-vms

Provide dummy libraries for alpha-vms

alpha-dec-vms always loads a number of libraries, -limagelib,
-lstarlet, and -lsys$public_vectors.  When running the ld testsuite
without a full cross-build environment, those libraries are missing
and cause fails.  This patch provides dummies, and tidies default.exp
a little.

	* testsuite/config/default.exp: Provide dummy libraries for
	alpha-dec-vms.
	(compiler_supports): New proc.  Use it for compiler tests.
	* testsuite/lib/ld-lib.exp (default_ld_assemble): Don't die if
	subdir not set.
	(run_ld_link_tests): Pass LDFLAGS to ld.
	* testsuite/ld-checks/checks.exp (section_check): Likewise.
	* testsuite/ld-scripts/assert.exp: Likewise.
	* testsuite/ld-scripts/extern.exp: Likewise.
	* testsuite/ld-scripts/log2.exp: Likewise.
	* testsuite/ld-scripts/map-address.exp: Likewise.
	* testsuite/ld-scripts/script.exp: Likewise.
	* testsuite/ld-scripts/sizeof.exp: Likewise.