[patch][pr88920] Fix noisy check_effective_target_offload_gcn

[Andrew Stubbs]

My recent patch to permit GCN testing using an LLVM assembler and linker 
has caused some noise in the log files for libgomp testing.

There are lots of messages like this:
  fatal error: GCC is not configured to support amdgcn-unknown-amdhsa as
  offload target

These messages are harmless; they're caused by 
check_effective_target_offload_gcn discovering the answer is "no". 
However, there's more noise than I intended.

This patch adds caching so that the message will only occur once per 
test run.

OK to commit?

Also, is there a way to redirect the output so that the message does not 
show up at all, unless the verbosity level is raised?

Thanks

Andrew
Cache effective-target llvm_binutils result.

2019-01-21  Andrew Stubbs  <a...@codesourcery.com>

	gcc/testsuite/
	* lib/target-supports.exp: Cache result.

diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index c0df467..f233f10 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -9255,6 +9255,7 @@ foreach N {df} {
 
 # Return 1 if this target uses an LLVM assembler and/or linker
 proc check_effective_target_llvm_binutils { } {
-    return [expr { [istarget amdgcn*-*-*]
-		   || [check_effective_target_offload_gcn] } ]
+    return [check_cached_effective_target llvm_binutils {
+	      expr { [istarget amdgcn*-*-*]
+		     || [check_effective_target_offload_gcn] }}]
 }