Use better test for usable compiler in ld testsuite.

Use better test for usable compiler in ld testsuite.

The ld testsuite includes numerous tests that depend on being able to
compile and link programs with the C compiler.  Some of these tests
use [which $CC] to check for the presence of the compiler before
proceeding with the test, but run_ld_link_exec_tests and run_cc_link_tests
give ERRORs if compilation fails.  Also, even if $CC is defined and present,
it may not be usable due to missing libraries, etc.

This patch adds a new procedure check_compiler_available that attempts
to build an empty program and caches the result.  Uses of [which $CC]
are replaced with calls to this procedure, and run_ld_link_exec_tests
and run_cc_link_tests now also guard attempts to use $CC.

2019-04-24  Sandra Loosemore  <sandra@codesourcery.com>

	ld/
	* testsuite/config/default.exp: Use [check_compiler_available]
	instead of [which $CC].
	* testsuite/ld-auto-import/auto-import.exp: Likewise.
	* testsuite/ld-cygwin/exe-export.exp: Likewise.
	* testsuite/ld-elf/audit.exp: Likewise.
	* testsuite/ld-elf/compress.exp: Likewise.
	* testsuite/ld-elf/dwarf.exp: Likewise.
	* testsuite/ld-elf/elf.exp: Likewise.
	* testsuite/ld-elf/indirect.exp: Likewise.
	* testsuite/ld-elf/linux-x86.exp: Likewise.
	* testsuite/ld-elf/shared.exp: Likewise.
	* testsuite/ld-elf/tls.exp: Likewise.
	* testsuite/ld-elf/wrap.exp: Likewise.
	* testsuite/ld-elfcomm/elfcomm.exp: Likewise.
	* testsuite/ld-elfvers/vers.exp: Likewise.
	* testsuite/ld-elfvsb/elfvsb.exp: Likewise.
	* testsuite/ld-elfweak/elfweak.exp: Likewise.
	* testsuite/ld-gc/gc.exp: Likewise.
	* testsuite/ld-i386/i386.exp: Likewise.
	* testsuite/ld-i386/no-plt.exp: Likewise.
	* testsuite/ld-i386/tls.exp: Likewise.
	* testsuite/ld-ifunc/ifunc.exp: Likewise.
	* testsuite/ld-mn10300/mn10300.exp: Likewise.
	* testsuite/ld-pe/pe-compile.exp: Likewise.
	* testsuite/ld-pe/pe-run.exp: Likewise.
	* testsuite/ld-pe/pe-run2.exp: Likewise.
	* testsuite/ld-pie/pie.exp: Likewise.
	* testsuite/ld-plugin/lto.exp: Likewise.
	* testsuite/ld-plugin/plugin.exp: Likewise.
	* testsuite/ld-scripts/crossref.exp: Likewise.
	* testsuite/ld-sh/sh.exp: Likewise.
	* testsuite/ld-shared/shared.exp: Likewise.
	* testsuite/ld-size/size.exp: Likewise.
	* testsuite/ld-srec/srec.exp: Likewise.
	* testsuite/ld-undefined/undefined.exp: Likewise.
	* testsuite/ld-unique/unique.exp: Likewise.
	* testsuite/ld-x86-64/mpx.exp: Likewise.
	* testsuite/ld-x86-64/no-plt.exp: Likewise.
	* testsuite/ld-x86-64/tls.exp: Likewise.
	* testsuite/ld-x86-64/x86-64.exp: Likewise.
	* testsuite/lib/ld-lib.exp (run_ld_link_exec_tests): Call
	check_compiler_available before trying to use the compiler.
	(run_cc_link_tests): Likewise.
	(check_compiler_available): New.  Use it instead of [which $CC].