compiletest: Use JSON "binary-format" to decide //@ only-elf and //@ ignore-elf

[Zalathar]

Some tests only apply to ELF targets, or want to enable different test revisions for ELF and non-ELF targets. So compiletest supports the //@ only-elf and //@ ignore-elf directives to make that possible.

Historically, deciding whether the current target is an ELF target relied on a handful of ad-hoc string comparisons against the target tuple.

However, nowadays compiletest has access to the output of --print=all-target-specs-json, and that JSON output specifies the binary format of each target, making it much easier to determine whether a target is ELF or not.

[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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