[ELF] Don't emit empty PT_LOAD segment

[evgeny777]

Summary

This can happen when:

• Linker script puts RW section first.
• There is no place for program headers.

ELF/Writer.cpp

for (Phdr &P : Phdrs) {
     // The TLS pointer goes after PT_TLS. At least glibc will align it,
     // so round up the size to make sure the offsets are correct.
     if (H.p_type == PT_TLS) {
       Out<ELFT>::TlsPhdr = &H;
       if (H.p_memsz)
         H.p_memsz = alignTo(H.p_memsz, H.p_align);
     }
   }
+
+  Phdrs.erase(
+      std::remove_if(Phdrs.begin(), Phdrs.end(),
+                     [](Phdr &P) { return !P.First && P.H.p_type == PT_LOAD; }),
+      Phdrs.end());
 }
 
 template <class ELFT> static typename ELFT::uint getEntryAddr() {
   if (Symbol *S = Config->EntrySym)
     return S->body()->getVA<ELFT>();
   return Config->EntryAddr;
 }
 

test/ELF/linkerscript/empty-load.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { .rw : { *(.rw) } .text : { *(.text) }  }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -private-headers %t1 | FileCheck %s
+
+## We expect 2 PT_LOAD segments
+# CHECK:       PHDR
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  LOAD
+# CHECK-NEXT:     filesz
+# CHECK-NEXT:  STACK
+# CHECK-NEXT:     filesz
+
+.globl _start
+_start:
+  jmp _start
+
+.section .rw, "aw"
+ .quad 0