cmake: Add quotes to handle missing ZEPHYR_TOOLCHAIN_VARIANT variable

If this environment variable is missing, the user gets a rather
confusing cmake error message about "STREQUAL" "gccarmemb" from the
lines below.

Adding quotes to the check makes cmake handle it better and instead
report the problem in the later ASSERT.

Signed-off-by: Björn Stenberg <bjorn@haxx.se>