fix maymorestack gcflag hook instrumentation

[cpunion]

Summary

• Parse -gcflags=-d=maymorestack=... and carry the hook into the llgo build config.
• Emit the mayMoreStack hook at ordinary Go function entry for the hook's package, skipping synthetic initializers and the hook function itself.
• Add stable test/go CLI coverage and remove only the covered maymorestack.go GOROOT xfails.

Coverage

Covered:

• test/go builds and runs a temporary program with -gcflags=-d=maymorestack=main.mayMoreStack and checks the hook count.
• Focused GOROOT maymorestack.go passes locally for Go 1.24.11, 1.25.0, and 1.26.0.
• Parser coverage includes direct, pattern-prefixed, space-separated, and comma-separated -d forms.

Not covered / intentionally out of scope:

• caller/debugline metadata, nil, channel, print, recover, GC/liveness/finalizer/goroutine work.
• Runtime package changes; this PR only adds compiler-side hook instrumentation and CLI flag plumbing.
• Local focused GOROOT runs were on darwin/arm64; linux coverage is left to PR CI.

Tests

• go test ./cmd/internal/flags -count=1
• go test ./cmd/internal/build -count=1
• go test ./cl -count=1
• go test ./test/go -count=1
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.24.11 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 45s -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.25.0 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 45s -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.26.0 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 45s -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.24.11 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -run-timeout 45s -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.25.0 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -run-timeout 45s -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -args -goroot /Users/lijie/sdk/go1.26.0 -dirs . -case '^maymorestack\\.go$' -directive-mode ci -run-timeout 45s -build-timeout 5m
• go test ./cmd/internal/... -count=1
• go test ./ssa -count=1
• go test ./internal/build -count=1

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 39.39394% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	39.39%	20 Missing ⚠️

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