scripts: Probe external toolchains for libthread-db

libthread-db is a package that can be configured for external
toolchains, so let's have the script probe for it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

scripts/ext-toolchain.sh

@@ -35,6 +35,7 @@ LIB_SPECS="
 	rt:       librt-* librt
 	pthread:  libpthread-* libpthread
 	stdcpp:   libstdc++
+	thread_db: libthread-db
 	gcc:      libgcc_s
 	ssp:      libssp
 	gfortran: libgfortran