errchk: fix build - allow matches in multiline error messages

TBR=r
CC=golang-dev
https://golang.org/cl/1734042

test/errchk

@@ -31,7 +31,12 @@ close(SRC);
 # Run command
 $cmd = join(' ', @ARGV);
 open(CMD, "exec $cmd </dev/null 2>&1 |") || die "BUG: errchk: run $cmd: $!";
-@out = grep { !/^	/ } <CMD>;
+
+# 6g error messages continue onto additional lines with leading tabs.
+# Split the output at the beginning of each line that doesn't begin with a tab.
+$out = join('', <CMD>);
+@out = split(/^(?!\t)/m, $out);
+
 close CMD;
 
 if($? == 0) {