test: fix ABI mismatch in fixedbugs/issue19507

Because run.go doesn't pass the package being compiled to the compiler
via the -p flag, it can't match up the main·f symbol from the
assembler with the "func f" stub in Go, so it doesn't produce the
correct assembly stub.

Fix this by removing the package prefix from the assembly definition.

Alternatively, we could make run.go pass -p to the compiler, but it's
nicer to remove these package prefixes anyway.

Should fix the linux-arm builder, which was broken by the introduction
of function ABIs in CL 147160.

Updates #27539.

Change-Id: Id62b7701e1108a21a5ad48ffdb5dad4356c273a6
Reviewed-on: https://go-review.googlesource.com/c/149483
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue19507.dir/div_arm.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT main·f(SB),0,$0-8
+TEXT ·f(SB),0,$0-8
 	MOVW	x+0(FP), R1
 	MOVW	x+4(FP), R2
 	DIVU	R1, R2