Add ElfSymbolModule for Parsing ELF Symbol Tables

[brianrob]

Summary

Adds ElfSymbolModule, an ELF (Executable and Linkable Format) symbol parser that resolves RVAs to symbol names. This enables PerfView and TraceEvent to resolve native symbols from Linux ELF binaries when analyzing universal traces captured on Linux.

Key capabilities

• Parses both .symtab and .dynsym sections
• Supports 32-bit and 64-bit ELF, little-endian and big-endian
• Implements ISymbolLookup for integration with TraceEvent's symbol resolution pipeline
• Demangles Itanium C++ (_Z) and Rust v0 (_R) mangled names using demanglers from Implement Symbol Demanglers for Linux Binaries #2383.

Design

• Lazy name resolution: Symbol names are decoded from the string table on first FindNameForRva hit, not during construction. Most symbols in a trace are never looked up — this avoids unnecessary work.
• LOH-free strtab: String table data retained in a SegmentedList<byte> with 64KB segments.
• Pre-allocated structures: Two-pass section header scan measures sizes before allocating — zero resizes.
• BitConverter fast path: 64-bit little-endian symbol entries parsed directly from byte arrays.
• O(log n) lookups: Sorted array with binary search.

Performance

To give a sense of existing vs. new performance of symbol parsing and lookup.

ELF vs PDB comparison (CoreCLR, 18K symbols, net8.0)

	ELF (ElfSymbolModule)	PDB (NativeSymbolModule / DIA)
Parse	2.3 ms / 3.0 MB	7.4 ms / 9.6 KB
Lookup (cold)	29 ns / 0 B	8,745 ns / 240 B

ELF parses 3.2x faster than PDB and first lookups are 303x faster (sorted array binary search vs COM interop). PDB's lower parse memory is due to DIA's lazy/deferred loading model.

Lookup benchmarks (zero-alloc)

Symbols	Lookup Time
5	7.6 ns
256	15.3 ns
10,772	21.7 ns
18,030	29.0 ns

Testing

Unit tests (26 tests)

Synthetic ELF binaries generated by ElfBuilder (no checked-in test binaries):

• Error handling: invalid magic, truncated, empty, bad ELF class, no section headers
• Format coverage: 64-bit LE, 32-bit LE, 64-bit BE, 32-bit BE
• Symbol filtering: non-function types, zero-value, zero-size, below PT_LOAD base
• RVA adjustment with pVaddr and pOffset
• Demangling integration: Itanium C++, Rust v0, plain passthrough
• .dynsym section parsing alongside .symtab
• Edge cases: empty table, RVA zero, 100-symbol binary search stress test
• File path constructor validation

Offline validation against pyelftools (568 real ELF files, 109,293 symbols)

Validated the parser against a known-good reference:

1. A Python script using pyelftools to extract the ground truth from 568 real .debug files: the executable PT_LOAD segment parameters and all STT_FUNC symbols (st_value, st_size, raw mangled name).
2. The C# test constructs an ElfSymbolModule with demangling disabled for each file, then verifies that every reference symbol is found at the correct RVA with an exact match on start address and raw name.
3. Multiple symbols sharing the same start RVA (GCC destructor D1/D2 aliases, LTO clones, ICF-merged functions) are handled by accepting any matching name at that address.

Result: zero mismatches across all 568 files and 109,293 symbols.

Contributes to #2382.