Add bootstrap step for stdarch-verify

[xonx4l]

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This PR hooks library/stdarch/crates/stdarch-verify crate into the bootstrap test runner as a step, so that we can run the stdarch-verify suite via x.py test.

Changes :
-> Added StdarchVerify in src/bootstrap/src/core/build_steps/test.rs
-> Added test::StdarchVerify in src/bootstrap/src/core/builder/mod.rs

Tests:
-> Running ./x.py test library/stdarch/crates/stdarch-verify builds compiler and std successfully.
-> All three integration tests Passes:
tests/arm.rs
tests/mips.rs
tests/x86-intel.rs
-> Doc tests for stdarch_verify also Passes.

r? @Kobzol