newlib: workaround cflags issue in crosstool for newlib-nano

When building newlib-nano because its treated as a companion_libs the
order it gets built ends up not having CT_ARCH_TARGET_CFLAGS set
correctly (as it gets cleared by a previous build stage).

This causes issues on ARM, so to workaround this we mimic the setting
of CT_ARCH_TARGET_CFLAGS in CT_LIBC_NEWLIB_NANO_TARGET_CFLAGS to
"-mthumb-interwork" (That mimics how CT_ARCH_TARGET_CFLAGS is set
because we enable CT_ARCH_ARM_INTERWORKING=y).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

configs/arm.config

@@ -46,6 +46,7 @@ CT_NEWLIB_NANO_DEVEL_URL="https://github.com/zephyrproject-rtos/newlib-cygwin.gi
 CT_NEWLIB_NANO_DEVEL_REVISION="4f5997d3c0f9011135e9627dad700c9d64be4a4b"
 CT_NEWLIB_NANO_GCC_LIBSTDCXX=y
 CT_NEWLIB_NANO_INSTALL_IN_TARGET=y
+CT_LIBC_NEWLIB_NANO_TARGET_CFLAGS="-mthumb-interwork"
 # CT_LIBC_NEWLIB_NANO_IO_C99FMT is not set
 # CT_LIBC_NEWLIB_NANO_IO_LL is not set
 CT_LIBC_NEWLIB_NANO_IO_FLOAT=y