[release/6.0] Fix KeepNativeSymbols to work on mono as well (backport of #66594)

[ayakael]

Backport of #66594

Per @omajid:

When packaging .NET on s390x through source-build, we want to keep debug
symbols enabled in the build. The package manager (debbuild, rpmbuild,
etc) generally strips the binaries to create distro-standard
-debug/-debuginfo packages. This was supported in coreclr via
--keepnativesymbols flag, but wasn't supported in mono (ie, s390x). Fix
that.

Customer impact

Source-build with mono-flavored runtime is problematic without this patch.

Testing

• unit tests in ci
• source-build build within Alpine environment passes

Risk

Low, as it is already in main, and only activates when KeepNativeSymbols!=true

[carlossanlop]

@marek-safar @directhex this needs to go through Tactics. When/if this is ready, please add the servicing-consider label and send an email to Tactics requesting merge approval.

[carlossanlop]

@omajid @ayakael @directhex I see some CI failures that I haven't seen in any other backport PR. Are they related to this change?

@directhex can you please provide a code review sign-off?

[ayakael]

@omajid @ayakael @directhex I see some CI failures that I haven't seen in any other backport PR. Are they related to this change?

@directhex can you please provide a code review sign-off?

It really shouldn't be, as KeepNativeSymbols is really only used by source-build from memory, and the failures seem related to CoreCLR which this PR doesn't touch. Pushing a new rebase as to trigger another round of CI tests. Maybe it was just a fluke.

[carlossanlop]

Ping @directhex @marek-safar for a code review-sign off.

[carlossanlop]

CI failure confirmed offline to be unrelated. Signed off by area owners. Approved by Tactics. No OOB package authoring changes needed. Ready to merge.