fix amd64 sysv abi to pass asan everywhere

I verified the PR by running the entire test suite of Odin itself with
`-sanitize:address` and also the ols test suite (which caused unique problems
before).

A test has also been added with some problematic code, Windows seems to
have problems with asan in CI or in general so it is not ran there.

The LB_ABI_COMPUTE_RETURN_TYPES block has been removed entirely because
it was unused, I got pretty confused why it didn't effect anything at
first.

Fixes #3211

tests/internal/Makefile

@@ -1,6 +1,6 @@
 ODIN=../../odin
 
-all: rtti_test map_test pow_test 128_test
+all: rtti_test map_test pow_test 128_test asan_test
 
 rtti_test:
 	$(ODIN) run test_rtti.odin -file -vet -strict-style -o:minimal
@@ -13,3 +13,6 @@ pow_test:
 
 128_test:
 	$(ODIN) run test_128.odin -file -vet -strict-style -o:minimal
+
+asan_test:
+	$(ODIN) run test_asan.odin -file -sanitize:address -debug

tests/internal/test_asan.odin

@@ -0,0 +1,62 @@
+// Intended to contain code that would trigger asan easily if the abi was set up badly.
+package test_asan
+
+import "core:fmt"
+import "core:testing"
+import "core:os"
+
+TEST_count := 0
+TEST_fail  := 0
+
+when ODIN_TEST {
+	expect  :: testing.expect
+	log     :: testing.log
+} else {
+	expect  :: proc(t: ^testing.T, condition: bool, message: string, loc := #caller_location) {
+		TEST_count += 1
+		if !condition {
+			TEST_fail += 1
+			fmt.printf("[%v] %v\n", loc, message)
+			return
+		}
+	}
+	log     :: proc(t: ^testing.T, v: any, loc := #caller_location) {
+		fmt.printf("[%v] ", loc)
+		fmt.printf("log: %v\n", v)
+	}
+}
+
+main :: proc() {
+	t := testing.T{}
+
+	test_12_bytes(&t)
+	test_12_bytes_two(&t)
+
+	fmt.printf("%v/%v tests successful.\n", TEST_count - TEST_fail, TEST_count)
+	if TEST_fail > 0 {
+		os.exit(1)
+	}
+}
+
+@(test)
+test_12_bytes :: proc(t: ^testing.T) {
+	internal :: proc() -> (a, b: f32, ok: bool) {
+		return max(f32), 0, true
+	}
+
+	a, b, ok := internal()
+	expect(t, a == max(f32), fmt.tprintf("a (%v) != max(f32)", a))
+	expect(t, b == 0,        fmt.tprintf("b (%v) != 0", b))
+	expect(t, ok,            fmt.tprintf("ok (%v) != true", ok))
+}
+
+@(test)
+test_12_bytes_two :: proc(t: ^testing.T) {
+	internal :: proc() -> (a: f32, b: int) {
+		return 100., max(int)
+	}
+
+	a, b := internal()
+	expect(t, a == 100.,     fmt.tprintf("a (%v) != 100.", a))
+	expect(t, b == max(int), fmt.tprintf("b (%v) != max(int)", b))
+}