cmd/asm: improve assembler error messages
    
    Provide file/line numbers for errors when we have them.
    Make the assembler error text closer to the equivalent errors from the compiler.
    
    Abort further processing when we come across errors.
    Fixes #53994
    
    Change-Id: I4d6a037d6d713c1329923fce4c1189b5609f3660
    Reviewed-on: https://go-review.googlesource.com/c/go/+/455276
    Reviewed-by: Keith Randall <khr@google.com>
    Run-TryBot: Keith Randall <khr@google.com>
    Auto-Submit: Keith Randall <khr@google.com>
    Reviewed-by: Cherry Mui <cherryyz@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>