cmd/internal/objfile: make lookupFunc an alias type

cmd/internal/objfile: make lookupFunc an alias type

In the x/arch repo, CL 45098 introduced SymLookup type, replacing
the unnamed function type for lookup functions. This affects the
signature of x86asm.GoSyntax. In particular, it cannot convert
one named type, namely lookupFunc, to the other without an
explicit cast. Make lookupFunc unnamed to fix.

Change-Id: I973300d29ef1dbfdbd7fc2429e89c5849e6a7329
Reviewed-on: https://go-review.googlesource.com/80842
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>