test: make codegen tests work with both ABIs

Some codegen tests were written with the assumption that
arguments and results are in memory, and with a specific stack
layout. With the register ABI, the assumption is no longer true.
Adjust the tests to work with both cases.

- For tests expecting in memory arguments/results, change to use
  global variables or memory-assigned argument/results.

- Allow more registers. E.g. some tests expecting register names
  contain only letters (e.g. AX), but  it can also contain numbers
  (e.g. R10).

- Some instruction selection changes when operate on register vs.
  memory, e.g. ADDQ vs. LEAQ, MOVB vs. MOVL. Accept both.

TODO: mathbits.go and memops.go still need fix.
Change-Id: Ic5932b4b5dd3f5d30ed078d296476b641420c4c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/309335
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

test/codegen/stack.go

@@ -92,11 +92,11 @@ func ArrayInit(i, j int) [4]int {
 // Check that assembly output has matching offset and base register
 // (issue #21064).
 
-func check_asmout(a, b int) int {
+func check_asmout(b [2]int) int {
 	runtime.GC() // use some frame
 	// amd64:`.*b\+24\(SP\)`
 	// arm:`.*b\+4\(FP\)`
-	return b
+	return b[1]
 }
 
 // Check that simple functions get promoted to nosplit, even when