[compiler-rt] Fix: windows i386 builtins tests (PR #179062)

[via llvm-commits]

thulasiramk-2310 wrote:

On Linux, i386 builtins may be built but the tests are typically not run by default, and when they are, the architecture is enforced explicitly via flags like ```-m32```.
On Windows with ```clang-cl```, there is no implicit ```-m32``` equivalent. The compiler instead infers the target from the MSVC environment, which defaults to x64 on 64-bit hosts, even when testing i386 runtimes.

Adding ```--target=i386-pc-windows-msvc``` makes the test target explicit on Windows, matching what ```-m32``` does on Linux and ensuring the tests run with the same architecture as the builtins under test.

https://github.com/llvm/llvm-project/pull/179062