cmd/link: improve gap detection in TestPIESize

In CL 210180 we detect gaps between PT_LOAD segments and subtract
them from size calculation. The code there only works when
PT_LOAD segments are next to each other. But it is possible that
there are other segments in between (e.g. a GNU_RELRO segment).
Relax the gap detection to count gaps between PT_LOAD segments
regardless of whether they are next to each other.

Updates #36023.
Updates #35545.

Change-Id: I8b94506359fa649a4478acc742d86d4b16022dbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/220654
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: cherrymui

src/cmd/link/elf_test.go

@@ -387,15 +387,15 @@ func TestPIESize(t *testing.T) {
 					}
 				}
 				// also skip gaps between PT_LOAD segments
-				for i := range ef.Progs {
-					if i == 0 {
+				var prev *elf.Prog
+				for _, seg := range ef.Progs {
+					if seg.Type != elf.PT_LOAD {
 						continue
 					}
-					p1 := ef.Progs[i-1]
-					p2 := ef.Progs[i]
-					if p1.Type == elf.PT_LOAD && p2.Type == elf.PT_LOAD {
-						ret += p2.Off - p1.Off - p1.Filesz
+					if prev != nil {
+						ret += seg.Off - prev.Off - prev.Filesz
 					}
+					prev = seg
 				}
 				return ret
 			}