Move libstdc++-nox out of IRAM (esp8266#5492)

The new non-exception libstdc++ was not referenced in the linker script,
allowing it to end up in IRAM when not needed.  Add the line to match
and move it into IROM where it belongs.

tools/sdk/ld/eagle.app.v6.common.ld.h

@@ -111,6 +111,7 @@ SECTIONS
     *libgcc.a:_umoddi3.o(.literal .text)
     *libgcc.a:_udivdi3.o(.literal .text)
     *libstdc++.a:( .literal .text .literal.* .text.*)
+    *libstdc++-nox.a:( .literal .text .literal.* .text.*)
     *libsmartconfig.a:(.literal .text .literal.* .text.*)
     *liblwip_gcc.a:(.literal .text .literal.* .text.*)
     *liblwip_src.a:(.literal .text .literal.* .text.*)