gcc-runtime: Use --with-target-subdir for baremetal targets

This disables tests for runtime system specific functions like fcntl
there is no other change seen

+configure:77372: checking for fcntl
+configure:77372: result: yes

In libsttdc++ configure log outputs.

[YOCTO #14803]

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>