bare-arm, stmhal, teensy: Duplicate -nostdlib to CFLAGS

As we are building with -nostdlib gcc features like the stack protector
will fail linking, because the failure handlers are in gcc's internal
libs. Such features are implicitly disabled during compilation when
-nostdlib is used in CFLAGS too.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>