panic_unwind: Use global_asm! for IMGREL relocations

[Darksonn]

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Rust consteval cannot construct 32-bit values that contain the offset between two symbols, but this is required for panic_unwind on some architectures because unwinding on Windows SEH uses image-base-relative pointers for the panic information (which makes them fit in 32-bit rather than 64-bit).

Currently, this is worked around by initializing these globals on panic with atomic stores. This works, but is undefined behavior as the panic runtime reads the values with non-atomic loads, which leads to a data race.

Instead, utilize global_asm! to create the globals with @IMGREL relocations so that the linker constructs the relative pointers for us at link or load time.

For context: #t-compiler > relative pointers in windows SEH panic_unwind @ 💬

AI assistance was involved with writing the assembly code.

[rustbot]

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[Fulgen301]

(Nitpick: I'd clarify that they're offsets from the image base and not __ImageBase, even if that's the exact same address.)

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[LawnGnome]

Re-rolling, as my asm is absolutely not good enough for this.

@rustbot reroll

[Mark-Simulacrum]

Definitely not familiar with the Windows aspects of this... but happy to approve given Bjorn has taken a look with a response to the comment (either telling me I'm wrong or changing things :)

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[Mark-Simulacrum]

@bors try jobs=msvc

[rust-bors]

💔 Test for ee5da48 failed: CI. Failed jobs:

• try - aarch64-msvc-1 (web logs, enhanced plaintext logs)
• try - aarch64-msvc-2 (web logs, enhanced plaintext logs)

[Darksonn]

@bors try jobs=msvc

[rust-bors]

💔 Test for aa22e3c failed: CI. Failed jobs:

• try - aarch64-msvc-1 (web logs, enhanced plaintext logs)
• try - aarch64-msvc-2 (web logs, enhanced plaintext logs)
• try - dist-aarch64-msvc (web logs, enhanced plaintext logs)

[Darksonn]

@bors try jobs=msvc

[rust-bors]

☀️ Try build successful (CI)
Build commit: 976338d (976338db6bad68e99abbdc7370002f113822c140)
Base parent: 7088e4b (7088e4b63a9516ebfbfe2ab2d999cf01a528ac14)

[rust-bors]

☔ The latest upstream changes (presumably #161256) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[Mark-Simulacrum]

r=me with rebase (and maybe commits squashed a bit)