runtime: add line info for stack frames

[cpunion]

Summary

• add native PC symbolization for runtime.Caller, CallersFrames, FuncForPC, and runtime.Stack
• preserve non-signal panic PCs so debug.Stack can report panic-site file:line
• enable line-info debug locations by default and emit dSYM on Darwin host builds
• keep debug-only SSA values from dropping required nil/bounds checks
• preserve single-case select receive assignment ordering when line info adds DebugRef instructions
• add stable test/go coverage for Caller/CallersFrames/FuncForPC/debug.Stack
• remove Darwin xfail entries for fixedbugs issue22083, issue29735, and issue7690

Tests

• go test ./test/go -run TestRuntimeLineInfoAndStack -count=1
• go test ./internal/build -count=1
• go test ./cl -run 'TestCompile|TestFromTestgo|TestRunAndTestFromTestgo' -count=1
• go test ./test/go -count=1
• go test ./test/goroot -run TestGoRoot -goroot /opt/homebrew/Cellar/go/1.26.0/libexec -dirs fixedbugs -case 'fixedbugs/(issue22083|issue29735|issue7690)\.go' -count=1

Notes

• left Linux/versioned xfails in place
• skipped GC/liveness/finalizer/goroutine and defer/recover semantic cases

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	94.11%	1 Missing and 1 partial ⚠️

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