cmd/link: skip R_ADDR relocs in .rela.plt for internal PIE

ld.addpltsym adds an R_X86_64_JMP_SLOT dynamic relocation to .rela.plt
and uses Addaddrplus to reference the GOT in Elf64_Rela.r_offset.

Addaddrplus results in an R_ADDR relocation, which here we transform
into an R_X86_64_64 dynamic relocation. This is wrong for several
reasons:

1. .rela.plt is not a writable, relro section. It is mapped read-only,
   causing the dynamic linker to segfault when it tried to handle the
   relocation. This was the immediate cause of internal PIE cgo
   crashes.

2. Relocations targetting other reloc sections are, as far as I can
   tell, undefined behavior in the ELF spec and are unlikely to be a
   good idea.

3. Even if the relocation did work, it isn't what we want. The
   relocation, if successfully handled, would have put an absolute
   address as the JMP_SLOT offset, but it should be the offset from the
   beginning of the binary, just like any other relocation. What we want
   is a statically resolved R_ADDR relocation, just as is used below for
   the R_X86_64_64 relocation.

Skipping the .rela.plt allows reloc() to handle these R_ADDR
relocations.

With this CL, internal PIE cgo binaries work.

Updates #18968

Change-Id: Ie74e6fe249e88150baa0e340b1cb128cf7f28673
Reviewed-on: https://go-review.googlesource.com/47837Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/amd64/asm.go

@@ -283,8 +283,15 @@ func adddynrel(ctxt *ld.Link, s *ld.Symbol, r *ld.Reloc) bool {
 			// now, or else we will be caught in an infinite loop
 			// of generating synthetic relocs for our synthetic
 			// relocs.
+			//
+			// Furthermore, the rela sections contain dynamic
+			// relocations with R_ADDR relocations on
+			// Elf64_Rela.r_offset. This field should contain the
+			// symbol offset as determined by reloc(), not the
+			// final dynamically linked address as a dynamic
+			// relocation would provide.
 			switch s.Name {
-			case ".dynsym", ".rela", ".got.plt", ".dynamic":
+			case ".dynsym", ".rela", ".rela.plt", ".got.plt", ".dynamic":
 				return false
 			}
 		} else {