compiletest/codegen-llvm: automatically add needs-target-std if needed

[xdoardo]

When running the codegen-llvm test suite, the needs-target-std directive is automatically added if and only if the #![no_std] line is not seen and the needs-target-std directive was not already seen.

This is a best-effort help to make running the codegen-llvm test suite for targets that do not support std slightly easier. Ideally, with time, the tests that aren't marked as no_std but in fact only use types from core/alloc will be patched to reflect this, so that they can be reused for targets without std.

For an explanation of this patch instead of a patch that marks all the tests that aren't #![no_std] with needs-target-std, see this Zulip thread. Let me know if, instead, a patch adding the needs-target-std is preferred.

A simple test of the introduced behaviour is ./x test tests/codegen-llvm/align-fn.rs --target=riscv32e-unknown-none-elf --force-rerun -v. The test does not have the needs-target-std directive, but it is ignored with this message:

test [codegen] tests/codegen-llvm/align-fn.rs ... ignored, ignored if target does not support std

[rustbot]

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[rustbot]

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[jieyouxu]

The idea seems okay to me, I'll think about the impl a bit tho, because "injecting synthesized directive" feels a tad bit hacky

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