Allow setting a custom memory limit during coverage report generation

[stze]

Allow setting -rss_limit_mb via the CLI

[louismerlin]

For honggfuzz:

 --rlimit_rss VALUE
    Per process RLIMIT_RSS in MiB (default: 0 [no limit]). It will also set *SAN's soft_rss_limit_mb if used

For AFL++:

  -m megs       - memory limit for child process (0 MB, 0 = no limit [default])

Should be simple to add, I only need to make sure to do the conversion between MB and MiB.