Ignore LLVM's blank lines.

Ignore LLVM's blank lines.

The GCN toolchain must use the LLVM assembler and linker because there's no
binutils port.  The LLVM tools do not have the same diagnostic style as
binutils, so the "blank line(s) in output" tests are inappropriate (and very
noisy).

The LLVM tools also have different command line options, so it's not possible
to autodetect object formats in the same way.

This patch addresses both issues.

2019-01-17  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* doc/sourcebuild.texi: Document dg-require-effective-target
	llvm_binutils and offload_gcn.

	gcc/testsuite/
	* lib/file-format.exp (gcc_target_object_format): Handle AMD GCN.
	* lib/gcc-dg.exp (gcc-dg-prune): Ignore blank lines from the LLVM
	linker.
	* lib/target-supports.exp (check_effective_target_llvm_binutils): New.

From-SVN: r268027