ctest: probe optional compiler flags

[chitao1234]

Description

Add TestGenerator::flag_if_supported so ctest callers can request C compiler flags that should only be passed when the selected compiler supports them.

This also changes ctest’s built-in warning suppressions for -Wno-address-of-packed-member and -Wno-unknown-warning-option to use cc::Build::flag_if_supported. Some older toolchains (e.g. the default one on dragonfly) reject them when -Werror is enabled.

The generated test compiler setup now sets the target before probing flags, so support checks use the same target configuration as the actual compile.

Sources

Not applicable.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI