cmake: Set minimum required Zephyr SDK version to 0.15

This commit sets the minimum required Zephyr SDK version to 0.15, which
is required to build the current Zephyr codebase due to the following
breaking changes:

* RISC-V ISA spec change to 20191213 requiring the Zicsr and Zifencei
  extensions to be specified to the `-march` flag
* ARC64 output format name change to `elf64-littlearc64`

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>