cmd/internal/objfile: don't require runtime.symtab symbol for XCOFF

For some reason (that I didn't look into), externally linked
AIX binaries don't have runtime.symtab symbol. Since recent Go
releases (Go 1.3 maybe?), that symbol is empty and not necessary
anyway. Don't require it.

Fixes #40972.

Change-Id: I73a1f0142195ea6debdba8a4f6e12cadc3980dc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/279995


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>