Enable static libgcc when CT_GDB_NATIVE_STATIC_LIBSTDCXX is set

This commit updates the GDB build script to specify `-static-libgcc`
when `CT_GDB_NATIVE_STATIC_LIBSTDCXX` is enabled. Both libgcc and
libstdc++ are considered to be part of the "standard libraries," and
should be specified by the same flag (the configuration symbol could
potentially use a better name and/or further indirection).

This also semantically aligns the `CT_GDB_NATIVE_STATIC_LIBSTDCXX`
with the equivalent GCC configuration `CT_CC_GCC_STATIC_LIBSTDCXX`,
which also enables static linking of both libgcc and libstdc++.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

scripts/build/debug/300-gdb.sh

@@ -237,6 +237,7 @@ do_gdb_backend()
         ldflags+=" -static"
     fi
     if [ "${static_libstdcxx}" = "y" ]; then
+        ldflags+=" -static-libgcc"
         ldflags+=" -static-libstdc++"
     fi