[dev.link] cmd/link: define _etext, etc. in the linker on Solaris

[dev.link] cmd/link: define _etext, etc. in the linker on Solaris

On Solaris, in the runtime it defines the external name of
runtime.etext as _etext (runtime/os3_solaris.go:13). In CL 224939
we changed to put external names in the ELF symbol table more
consistently. In this case it will contain _etext but not
runtime.etext.

To be conservative, this CL defines both runtime.etext and _text
in the linker.

Change-Id: I79f196e87b655042be97b0fbbab02d0ebc8db2fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/225537
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>