Convert inline assembly sym operands into GCC input operands

This commit updates `<Builder as AsmBuilderMethods>::codegen_inline_asm`
to convert `sym` operands into `"X" (&func_or_static)` input operands
to indicate the dependency on the referenced symbols and prevent them
from being eliminated.

We follow the suit of the LLVM codegen with a mixture of its differing
techniques for `asm!` and `global_asm!`. The codegen module generates
input operands for the `sym` operands (as in `asm!` in cg_llvm).
However, the codegen module replaces all placeholders with mangled
symbol names before passing the assembly template string to the backend
(as in `global_asm!` in cg_llvm), which means these input operands are
never referenced in the final assembly template string.

Unlike the LLVM codegen, the input operand constraint must be `X`
instead of `s`. If the `s` constraint is used, GCC will employ checks to
make sure that the operand can really be represented by a simple
symbolic constant, thus rejecting symbols requiring GOT, etc. to
resolve. Such checks are unnecessary for Rust `sym` as it's up to
programmers to handle such complex cases, e.g., by manually appending
GOT addressing modifiers to the substituted symbol names.

Using the `X` constraint doesn't seem to generate any extra code, so
this will not compromise the property of naked functions.

Author: yvt

src/asm.rs

@@ -13,6 +13,7 @@ use std::borrow::Cow;
 use crate::builder::Builder;
 use crate::context::CodegenCx;
 use crate::type_of::LayoutGccExt;
+use crate::callee::get_fn;
 
 
 // Rust asm! and GCC Extended Asm semantics differ substantially.
@@ -343,9 +344,24 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     // processed in the previous pass
                 }
 
-                InlineAsmOperandRef::Const { .. }
-                | InlineAsmOperandRef::SymFn { .. }
-                | InlineAsmOperandRef::SymStatic { .. } => {
+                InlineAsmOperandRef::SymFn { instance } => {
+                    inputs.push(AsmInOperand {
+                        constraint: "X".into(),
+                        rust_idx,
+                        val: self.cx.rvalue_as_function(get_fn(self.cx, instance))
+                            .get_address(None),
+                    });
+                }
+
+                InlineAsmOperandRef::SymStatic { def_id } => {
+                    inputs.push(AsmInOperand {
+                        constraint: "X".into(),
+                        rust_idx,
+                        val: self.cx.get_static(def_id).get_address(None),
+                    });
+                }
+
+                InlineAsmOperandRef::Const { .. } => {
                     // processed in the previous pass
                 }
             }