cg_llvm: Clean up some inline assembly bindings
This PR combines a few loosely-related cleanups to LLVM bindings related to inline assembly. These include:
- Replacing `LLVMRustInlineAsm` with LLVM-C's `LLVMGetInlineAsm`
- Adjusting FFI declarations to avoid the need for explicit `as_c_char_ptr` conversions
- Flattening control flow in `inline_asm_call`
There should be no functional changes.