Issue deprecation warnings for safe accesses to extern statics

[petrochenkov]

Fixes#35112
cc #36247

[rust-highfive]

r? @nikomatsakis

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[nikomatsakis]

the new policy is to make a separate tracking issue, following roughly this template; if you could update with an issue like that, it'd be great.

[petrochenkov]

@nikomatsakis
Updated.

[nikomatsakis]

Looks great. =)

[nikomatsakis]

@bors r+

[bors]

📌 Commit ef489fc has been approved by nikomatsakis

[bors]

⌛ Testing commit ef489fc with merge 4391d8e...

[bors]

💔 Test failed - auto-win-msvc-64-opt-rustbuild

[petrochenkov]

@bors r=nikomatsakis

[bors]

📌 Commit aadbcff has been approved by nikomatsakis

[bors]

⌛ Testing commit aadbcff with merge 3781956...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: nikomatsakis
Pushing 3781956 to master...