testsuite: coverage: fix -Wcast-align warning

[nmeum]

While porting the coverage.c file from RIOT to Zephyr (which employs different compiler flags) I noticed the following -Wcast-align GCC warnings on arm:

coverage.c: In function 'buff_write_u64':
coverage.c: error: cast increases required alignment of target type [-Werror=cast-align]
   64 |         *((uint32_t *)((uint8_t *)buffer + *off) + 0) = (uint32_t)(v & MASK_32BIT);
      |           ^
coverage.c: error: cast increases required alignment of target type [-Werror=cast-align]
   65 |         *((uint32_t *)((uint8_t *)buffer + *off) + 1) = (uint32_t)((v >> 32) &
      |           ^
coverage.c: In function 'buff_write_u32':
coverage.c: error: cast increases required alignment of target type [-Werror=cast-align]
   75 |         *((uint32_t *)((uint8_t *)buffer + *off)) = v;
      |           ^

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 byte-wise writes using memcpy. Proposing a backport of this change as I figured it might be useful for Zephyr as well. If you are not interested in this change feel free to close this PR.