runtime: clean up libc_* definitions on Solaris

runtime: clean up libc_* definitions on Solaris

All functions defined in syscall2_solaris.go have the respective libc_*
var in syscall_solaris.go, except for libc_close. Move it from
os3_solaris.go

Remove unused libc_fstat.

Order go:cgo_import_dynamic and go:linkname lists in
syscall2_solaris.go alphabetically.

Change-Id: I9f12fa473cf1ae351448ac45597c82a67d799c31
Reviewed-on: https://go-review.googlesource.com/97736
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>