Fuzzing and rust coverage

[catenacyber]

Link to redmine ticket:
https://redmine.openinfosecfoundation.org/issues/4125

Describe changes:

• do not rebuild rust std when instrumenting for coverage (as both seem incompatible)
• fuzz driver accepts directories as well as single files now

TL;DR
To get coverage report including rust with oss-fuzz, we will also need rustc changes, and oss-fuzz changes
To get this manually is possible, cf below

To get coverage, I compile with

export RUSTFLAGS="$RUSTFLAGS -Zinstrument-coverage"
export CFLAGS="-g -O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fprofile-instr-generate -fcoverage-mapping -pthread -fsanitize=fuzzer-no-link"
export CXXFLAGS="-g -O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fprofile-instr-generate -fcoverage-mapping -pthread -fsanitize=fuzzer-no-link -stdlib=libc++"

I also run in rust subdirectory

abspath=`cargo metadata | jq -r '.workspace_root'`
export RUSTFLAGS="$RUSTFLAGS --remap-path-prefix src=$abspath/src"

to get the absolute file paths for suricata's crate, so that llvm-cov can generate a nice report

You need some fresh rust nightly compiler and llvm 10
For LLVM 11 and 12, rust compiler is not ready yet cf rust-lang/rust#79365