Fix RISC-V Test Failures in ./x test for Multiple Codegen and Assembly Cases

[CaiWeiran]

This PR addresses and resolves several test failures that occurred when running ./x test on the RISC-V architecture. The issues were due to platform-specific behavior, ABI differences, or code generation inconsistencies specific to RISC-V.

The following tests have been fixed:

• assembly/dwarf-mixed-versions-lto.rs
• codegen/const-vector.rs
• codegen/enum/enum-aggregate.rs
• codegen/simd/extract-insert-dyn.rs
• codegen/transmute-scalar.rs
• codegen/uninhabited-transparent-return-abi.rs

All changes have been tested locally with ./x test on a RISC-V target and now pass as expected.

Notes:

• These fixes are scoped specifically to enable full test suite compliance for RISC-V targets.
• No changes were made that affect other architectures.

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[Mark-Simulacrum]

Why split this out to a separate file? Skimming the contents I don't see major differences vs the existing file. Maybe we could use revisions instead if there's a few small differences (see tests/codegen/sanitizer/kcfi/emit-kcfi-operand-bundle-itanium-cxx-abi-generalized.rs for example).

[CaiWeiran]

The difference lies in the // CHECK lines. Would it be possible to separate them using revisions?