x86: Correctly pass larger structs/types in registers with -Zregparm

Fixes #145694.

The -Zregparm option for 32-bit x86 modifies the calling convention to pass
a number of arguments in registers instead of on the stack. (This is primarily
used by the Linux kernel.)

Currently, rustc will only pass primitive integer types in registers, which
seems to match the gcc documentation[1], which says that arguments "of integral
type" are passed as registers. This also matches the fastcall and vectorcall
conventions on windows.

However, it seems that _any_ type — most particularly structs — should be
passed as a register if possible. This matches what clang and gcc do, and so
avoids an ABI mismatch when linking with C code (which, after all, is the whole
point of supporting -Zregparm).

Author: sulix

compiler/rustc_target/src/callconv/x86.rs

@@ -183,7 +183,9 @@ pub(crate) fn fill_inregs<'a, Ty, C>(
 
         free_regs -= size_in_regs;
 
-        if arg.layout.size.bits() <= 32 && unit.kind == RegKind::Integer {
+        if opts.flavor != Flavor::FastcallOrVectorcall
+            || arg.layout.size.bits() <= 32 && unit.kind == RegKind::Integer
+        {
             attrs.set(ArgAttribute::InReg);
         }