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

[CaiWeiran]

This PR resolves several test failures encountered when running ./x test on the RISC-V architecture. These failures were caused by platform-specific behavior, ABI differences, or codegen inconsistencies unique to RISC-V.

The following test cases have been fixed to ensure compatibility with RISC-V:

• codegen-llvm/enum/enum-match.rs
• codegen-llvm/enum/enum-transparent-extract.rs
• codegen-llvm/repeat-operand-zero-len.rs
• codegen-llvm/enum/enum-aggregate.rs
• codegen-llvm/uninhabited-transparent-return-abi.rs

In addition, this PR adjusts tests/codegen-llvm/transmute-scalar.rs to explicitly specify the target architecture:

//@ compile-flags: --target=x86_64-unknown-linux-gnu
//@ needs-llvm-components: x86

As suggested by @nikic, this test is not target-specific and already uses minicore, implying it is meant to run against a stable triple regardless of the host architecture. Explicitly setting the target ensures consistent codegen behavior, particularly when testing on non-x86 platforms such as riscv64.

All changes have been tested locally 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.
• No changes impact other architectures.
• The change to transmute-scalar.rs aligns with the intent of [#143915](Fix RISC-V Test Failures in ./x test for Multiple Codegen and Assembly Cases #143915) and prevents architecture-dependent discrepancies during CI or local testing.

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[Mark-Simulacrum]

r=me with commits squashed

[CaiWeiran]

r=me with commits squashedr=me，提交被压缩

@Mark-Simulacrum Thanks! Commits squashed as requested.