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.