cmd/link/internal/loadelf: set AttrExternal on text section symbols

PPC64 processes external object relocations against the section
symbols. This needs to be set correctly to determine the type of
PLT stub to generate when both Go and External code make PLT calls.

Change-Id: I5abdd5a0473866164083c33e80324dffcc1707f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/488895
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/link/internal/loadelf/ldelf.go

@@ -540,6 +540,7 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 		}
 		if sect.type_ == elf.SHT_PROGBITS {
 			sb.SetData(sect.base[:sect.size])
+			sb.SetExternal(true)
 		}
 
 		sb.SetSize(int64(sect.size))