compiletest: add implied needs-target-std for codegen mode tests unless annotated with #![no_std]/#![no_core]

[jieyouxu]

A codegen mode test (such as codegen-llvm test suite) will now by default have an implied //@ needs-target-std directive, unless the test explicitly has an #![no_std]/#![no_core] attribute which disables this behavior.

• When a test has both #![no_std]/#![no_core] and //@ needs-target-std, the explicit //@ needs-target-std directive will cause the test to be ignored for targets that do not support std still.

This is to make it easier to test out-of-tree targets / custom targets (and targets not tested in r-l/r CI) without requiring target maintainers to do a bunch of manual //@ needs-target-std busywork.

Context: #t-compiler/help > `compiletest` cannot find `core` library for target != host

Implementation remarks

This is an alternative version of #150672, with some differences:

• This PR applies this implied-needs-target-std behavior to all codegen test mode tests.
• This PR does the synthetic directive injection in the same place as implied-codegen-run directives. Both are of course hacks, but at least they're together next to each other.