toolchain-funcs.eclass: fix tc-ld-disable-gold when using clang

tc-ld-disable-gold checks gcc version to see if we have gcc-4.8+
The version check fails if clang is set as the compiler.
  $ clang -E -P - <<<"__GNUC__ __GNUC_MINOR__ __GNUC_PATCHLEVEL__"
  4 2 1
i.e. clang returns a gcc version of 4.2.1
This results in incorrectly adding -B ... to LDFLAGS, when clang
supports "-fuse-ld" just fine.

Support for "-fuse-ld" first appeared in clang-3.5, so check clang
version and use the flag if supported.

eclass/toolchain-funcs.eclass

@@ -391,11 +391,28 @@ tc-ld-disable-gold() {
 	local path_ld=$(which "${bfd_ld}" 2>/dev/null)
 	[[ -e ${path_ld} ]] && export LD=${bfd_ld}
 
-	# Set up LDFLAGS to select gold based on the gcc version.
-	local major=$(gcc-major-version "$@")
-	local minor=$(gcc-minor-version "$@")
-	if [[ ${major} -lt 4 ]] || [[ ${major} -eq 4 && ${minor} -lt 8 ]] ; then
-		# <=gcc-4.7 requires some coercion.  Only works if bfd exists.
+	# Set up LDFLAGS to select gold based on the gcc / clang version.
+	local fallback="true"
+	if tc-is-gcc; then
+		local major=$(gcc-major-version "$@")
+		local minor=$(gcc-minor-version "$@")
+		if [[ ${major} -gt 4 ]] || [[ ${major} -eq 4 && ${minor} -ge 8 ]]; then
+			# gcc-4.8+ supports -fuse-ld directly.
+			export LDFLAGS="${LDFLAGS} -fuse-ld=bfd"
+			fallback="false"
+		fi
+	elif tc-is-clang; then
+		local major=$(clang-major-version "$@")
+		local minor=$(clang-minor-version "$@")
+		if [[ ${major} -gt 3 ]] || [[ ${major} -eq 3 && ${minor} -ge 5 ]]; then
+			# clang-3.5+ supports -fuse-ld directly.
+			export LDFLAGS="${LDFLAGS} -fuse-ld=bfd"
+			fallback="false"
+		fi
+	fi
+	if [[ ${fallback} == "true" ]] ; then
+		# <=gcc-4.7 and <=clang-3.4 require some coercion.
+		# Only works if bfd exists.
 		if [[ -e ${path_ld} ]] ; then
 			local d="${T}/bfd-linker"
 			mkdir -p "${d}"
@@ -404,9 +421,6 @@ tc-ld-disable-gold() {
 		else
 			die "unable to locate a BFD linker to bypass gold"
 		fi
-	else
-		# gcc-4.8+ supports -fuse-ld directly.
-		export LDFLAGS="${LDFLAGS} -fuse-ld=bfd"
 	fi
 }