cmd/asm: initialize assembler before running error tests

The Test{ARCH}Errors tests will call ctxt.Arch.Assemble, but this
function requires the assembler has been initialized. So this CL adds
a call to architecture.Init(ctxt) in testErrors, otherwise running
Test{ARCH}Errors alone would fail.

Change-Id: I4f3ba5a5fc1375d28779701989cf700cb4d1b635
Reviewed-on: https://go-review.googlesource.com/c/go/+/505976
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Eric Fang <eric.fang@arm.com>

Author: erifan01

src/cmd/asm/internal/asm/endtoend_test.go

@@ -272,6 +272,7 @@ var (
 func testErrors(t *testing.T, goarch, file string, flags ...string) {
 	input := filepath.Join("testdata", file+".s")
 	architecture, ctxt := setArch(goarch)
+	architecture.Init(ctxt)
 	lexer := lex.NewLexer(input)
 	parser := NewParser(ctxt, architecture, lexer, false)
 	pList := new(obj.Plist)