Hi, about the possible lib gcc toolchain issue.

Of the available gcc.a, I am using the one suffixed with '#' when
using the sdk. Note that it seems that this is different on your
machine.

./fpu/libgcc.a
./armv6-m/libgcc.a
./libgcc.a
./thumb/libgcc.a
./armv7-m/libgcc.a
./armv7e-m/fpu/libgcc.a
./armv7e-m/fpu/fpv5-sp-d16/libgcc.a
./armv7e-m/fpu/fpv5-d16/libgcc.a
./armv7e-m/libgcc.a ##################
./armv7e-m/softfp/libgcc.a
./armv7e-m/softfp/fpv5-sp-d16/libgcc.a
./armv7e-m/softfp/fpv5-d16/libgcc.a

And when using 'ZEPHYR_TOOLCHAIN_VARIANT=gccarmemb' I get:

./libgcc.a
./thumb/v7e-m/fpv4-sp/softfp/libgcc.a
./thumb/v7e-m/fpv4-sp/hard/libgcc.a
./thumb/v7e-m/libgcc.a ##############
./thumb/v7e-m/fpv5/softfp/libgcc.a
./thumb/v7e-m/fpv5/hard/libgcc.a
./thumb/libgcc.a
./thumb/v8-m.main/libgcc.a
./thumb/v8-m.main/fpv5/softfp/libgcc.a
./thumb/v8-m.main/fpv5/hard/libgcc.a
./thumb/v8-m.main/fpv5-sp/softfp/libgcc.a
./thumb/v8-m.main/fpv5-sp/hard/libgcc.a
./thumb/v6-m/libgcc.a
./thumb/v7-ar/fpv3/softfp/libgcc.a
./thumb/v7-ar/fpv3/hard/libgcc.a
./thumb/v7-ar/libgcc.a
./thumb/v8-m.base/libgcc.a
./thumb/v7-m/libgcc.a
./hard/libgcc.a


Relevant commands:

grep LOAD b/zephyr/zephyr.map
find . -name libgcc.a

AFAIK it is possible to use ZEPHYR_TOOLCHAIN_VARIANT=gccarmemb on mac.
I would recommend this over ZEPHYR_TOOLCHAIN_VARIANT=xtools.

From the docs:

"In case a toolchain is not available for the board you are using, you can build a toolchain from scratch using crosstool-NG. Follow the steps on the crosstool-NG website to prepare your host"
"http://docs.zephyrproject.org/getting_started/installation_mac.html#using-a-3rd-party-toolchain"