Lingkai Dong authored Nov 03, 2022 and Jaeden Amero committed Nov 04, 2022

The latest CMSIS-DSP no longer requires us to
* set `SOURCE_SUBDIR` to `Source` because a top-level `CMakeLists.txt`
is now provided
* add `-flax-vector-conversions` which is now already applied in CMSIS-
DSP's own compile options for GCC.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>