Auto merge of #140914 - Zalathar:asm-bindings, r=compiler-errors

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.

Author: bors