[BOLT] Provide backwards compatibility for YAML profile with std::hash

[aaupov]

Provide backwards compatibility for YAML profile that uses std::hash:
xxh3 hash is the default for newly produced profile (sets std-hash: false),
whereas the profile that doesn't specify std-hash will be treated as
std-hash: true, preserving old behavior.

Same as profile-use-dfs in https://reviews.llvm.org/D156176.

[github-actions]

:white_check_mark: With the latest revision this PR passed the C/C++ code formatter.

[maksfb]

How about we write a new field to YAML with a hash function name/ID? In the reader, we default to std::hash for backwards compatibility as you already do. If we are not planning on using std::hash in the future (for writing), perhaps the option is not needed at all? We can likely have a couple of binary/YAML test cases that verify backwards compatibility.

Suggestion for the title: "Provide backwards compatibility for YAML profile with std::hash".

[aaupov]

We can likely have a couple of binary/YAML test cases that verify backwards compatibility

Added a YAML test checking backwards compatibility. It reuses the test bolt/test/X86/reader-stale-yaml.test and its input (bolt/test/X86/Inputs/blarge_profile_stale.yaml) from before xxh diff.

[aaupov]

Renamed Xxh3 to XXH3.
Added the Default enum value aliased to XXH3 to avoid confusion which hash to use.