testsuite: coverage: fix -Wcast-align warning

While porting the coverage.c file from RIOT to Zephyr, which employs
different compiler flags, I noticed several -Wcast-align GCC warnings on
arm. I think, as is, the current implementation may perform unaligned
memory accesses which may not be supported on certain platforms. To
workaround that, I have rewritten the code for RIOT using
bytewise-writes with `memcpy`.

Signed-off-by: Sören Tempel <tempel@uni-bremen.de>