meta-zephyr-core: gcc-cross: fix arch

Use correct architecture for cortex-m3 and m4.

Signed-off-by: Ettore Chimenti <ettore.chimenti@seco.com>

meta-zephyr-core/recipes-devtools/gcc/gcc-cross_6.%.bbappend

-EXTRA_OECONF:append:armv7m = " --with-arch=armv7-m --with-mode=thumb "
+EXTRA_OECONF:append:armv7em = " --with-arch=armv7-em --with-mode=thumb "