detect musl at compile time

[richardstartin]

What does this PR do?:
If confstr(_CS_GNU_LIBC_VERSION, NULL, 0) stops producing an error for whatever reason at any time in the future on any musl systems, we would not relocate pointers in the dynamic section correctly. We just don't need to detect musl at runtime, and can continue to rely on separate binaries for each libc for a number of reasons:

1. we have robust infrastructure to load the right dynamic library
2. each binary is very small (under 0.5MiB)
3. We have glibc symbol versioning to simplify targeting older glibc systems (which we would have to remove)

Given the above, detecting musl at compile time feels like a safer approach.

Motivation:

Additional Notes:

How to test the change?:

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[github-actions]

🔧 Report generated by pr-comment-scanbuild

Scan-Build Report

User:	runner@fv-az1020-841
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 clean all
Clang Version:	Ubuntu clang version 14.0.0-1ubuntu1.1
Date:	Fri May 10 11:59:40 2024

Bug Summary

Bug Type	Quantity	Display?
All Bugs	6
Logic error
Assigned value is garbage or undefined	1
Dereference of null pointer	2
Result of operation is garbage or undefined	1
Unused code
Dead initialization	1
Dead nested assignment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Assigned value is garbage or undefined	dwarf.cpp	parseInstructions	235	20
Unused code	Dead initialization	livenessTracker.cpp	cleanup_table	45	1
Unused code	Dead nested assignment	vmStructs.cpp	checkNativeBinding	818	1
Logic error	Dereference of null pointer	flightRecorder.cpp	flush	1410	8
Logic error	Dereference of null pointer	safeAccess.h	load	35	18
Logic error	Result of operation is garbage or undefined	dwarf.h	getSLeb	142	25

[github-actions]

🔧 Report generated by pr-comment-cppcheck

CppCheck Report

Warnings (5)

• Obsolete function 'alloca' called.
• Member variable 'ScopeDesc::_stream' is not initialized in the constructor.
• Member variable 'ScopeDesc::_method_offset' is not initialized in the constructor.
• Member variable 'ScopeDesc::_bci' is not initialized in the constructor.
• Member variable 'ElfParser::_vaddr_diff' is not initialized in the constructor.

Style Violations (161)

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