Improve build performance with simple hot-path optimizations

[cpunion]

Summary

This extracts the lower-risk, source-only build performance changes from the larger build-perf branch.

Included changes focus on simple hot-path reductions:

• cache repeated tool/sysroot/LLVM setup work
• make SSA sanity checking opt-in for normal builds
• reduce package load/type-check allocations
• skip parser object resolution for build loads
• avoid unused type scopes
• use upstream parallel go/ssa.Program.Build
• skip redundant SSA program builds
• avoid MethodSet construction in SSA order fixup
• reduce embed/type-conversion allocations
• avoid external go list -json for Plan9 asm discovery while preserving internal/chacha8rand stub behavior
• remove unused reflect/method metadata maps
• pre-size predictable linkMainPkg scratch containers
• cache parsed runtime Go minor version

Intentionally excluded from this PR:

• async LLVM/object emission changes
• stdin-based IR compilation
• build-cache publication/archive scheduling changes
• manifest writer/format optimizations
• CI workflow fan-out/sharding changes

The branch does not modify cache archive/manifest generation or cache publication ordering. The only cache-related behavior kept is adding LLGO_SSA_SANITY to build env inputs when sanity checking is explicitly enabled, so cached artifacts distinguish sanity mode from normal builds.

Local timing

Representative paired local runs on darwin/arm64, Go 1.24.11:

go test ./internal/build -run '^TestExtest$' -count=3

• main: 35.621s reported, 36.82s wall
• this PR: 32.356s / 32.359s reported, 32.88s / 32.91s wall

Approx wall improvement: ~10.6%.

go test ./internal/build -count=1

• main: 48.644s reported, 49.17s wall
• this PR: 43.259s / 44.594s reported, 43.78s / 45.19s wall

Approx wall improvement: ~8-11%.

Validation

• go test ./internal/packages ./internal/build ./internal/crosscompile ./internal/env ./internal/goembed ./ssa ./xtool/env/llvm -count=1
• go test ./internal/build -run '^TestExtest$' -count=3
• go test ./test/std/math/rand/v2 ./test/std/reflect -count=1
• go test ./internal/build -run '^TestPkgSFilesChacha8UsesStubBeforeOtherFiles$|^TestParallelObjectEmit|MainModule|Abi|Reflect' -count=1
• go build -a -tags=dev -o /tmp/llgo-build-perf-simple ./cmd/llgo

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.04673% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ssa/type_cvt.go	80.76%	7 Missing and 3 partials ⚠️
internal/goembed/goembed.go	66.66%	2 Missing and 2 partials ⚠️
internal/crosscompile/crosscompile.go	85.71%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[cpunion]

Closing per decision: CI/build-time benefit is not strong or stable enough to proceed with this build-performance PR.