llvmcall segfaults if an argument has size 0 #60063 fix

[Quantumcies]

This code change used AI (Gemini Pro 3) to aid in the bug fixing process.

The primary issue, #60063, llvmcall would segfault if a zero-sized type (like Nothing or an empty struct) was passed as an argument.
The fix in src/cgutils.cpp (_julia_struct_to_llvm) now correctly detects 0-size types and returns LLVM::VoidTy immediately, preventing the creation of invalid LLVM structures.

While working on this, I also discovered that the jl_temporary_root call in the same function was unsafe and caused build failures on strict GC backends (like MMTk).
I replaced it with JL_GC_PUSH1, which explicitly roots the value on the C stack. This means this PR also fixes a build-blocking garbage collection safety bug during sysimage generation.

Thank you for helping me improve the quality of my first contribution! Sorry if I caused any hassle earlier.

[vtjnash]

Sorry, but nothing Gemini did here looks valid