[yaml2obj][ELF] ELF Relocations Support.

The patch implements support for both relocation record formats: Elf_Rel
and Elf_Rela. It is possible to define relocation against symbol only.
Relocations against sections will be implemented later. Now yaml2obj
recognizes X86_64, MIPS and Hexagon relocation types.

Example of relocation section specification:
Sections:
- Name: .text
  Type: SHT_PROGBITS
  Content: "0000000000000000"
  AddressAlign: 16
  Flags: [SHF_ALLOC]

- Name: .rel.text
  Type: SHT_REL
  Info: .text
  AddressAlign: 4
  Relocations:
    - Offset: 0x1
      Symbol: glob1
      Type: R_MIPS_32
    - Offset: 0x2
      Symbol: glob2
      Type: R_MIPS_CALL16

The patch reviewed by Michael Spencer, Sean Silva, Shankar Easwaran.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206017 91177308-0d34-0410-b5e6-96231b3b80d8

test/Object/yaml2obj-elf-rel.yaml

@@ -0,0 +1,118 @@
+# RUN: yaml2obj -format=elf %s | llvm-readobj -sections -relocations - | FileCheck %s
+
+!ELF
+FileHeader: !FileHeader
+  Class: ELFCLASS32
+  Data: ELFDATA2MSB
+  Type: ET_REL
+  Machine: EM_MIPS
+
+Sections:
+- Name: .text
+  Type: SHT_PROGBITS
+  Content: "0000000000000000"
+  AddressAlign: 16
+  Flags: [SHF_ALLOC]
+
+- Name: .rel.text
+  Type: SHT_REL
+  Info: .text
+  AddressAlign: 4
+  Relocations:
+    - Offset: 0x1
+      Symbol: glob1
+      Type: R_MIPS_32
+    - Offset: 0x1
+      Symbol: glob2
+      Type: R_MIPS_CALL16
+    - Offset: 0x2
+      Symbol: loc1
+      Type: R_MIPS_LO16
+
+- Name: .rela.text
+  Type: SHT_RELA
+  Link: .symtab
+  Info: .text
+  AddressAlign: 4
+  Relocations:
+    - Offset: 0x1
+      Addend: 1
+      Symbol: glob1
+      Type: R_MIPS_32
+    - Offset: 0x1
+      Addend: 2
+      Symbol: glob2
+      Type: R_MIPS_CALL16
+    - Offset: 0x2
+      Addend: 3
+      Symbol: loc1
+      Type: R_MIPS_LO16
+
+Symbols:
+  Local:
+    - Name: loc1
+    - Name: loc2
+  Global:
+    - Name: glob1
+      Section: .text
+      Value: 0x0
+      Size: 4
+    - Name: glob2
+  Weak:
+    - Name: weak1
+
+# CHECK:        Section {
+# CHECK-NEXT:     Index: 0
+# CHECK:        }
+# CHECK:        Section {
+# CHECK-NEXT:     Index: 1
+# CHECK-NEXT:     Name: .text (1)
+# CHECK:        }
+# CHECK-NEXT:   Section {
+# CHECK-NEXT:     Index: 2
+# CHECK-NEXT:     Name: .rel.text (7)
+# CHECK-NEXT:     Type: SHT_REL (0x9)
+# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Address: 0x0
+# CHECK-NEXT:     Offset: 0x160
+# CHECK-NEXT:     Size: 24
+# CHECK-NEXT:     Link: 4
+# CHECK-NEXT:     Info: 1
+# CHECK-NEXT:     AddressAlignment: 4
+# CHECK-NEXT:     EntrySize: 8
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Section {
+# CHECK-NEXT:     Index: 3
+# CHECK-NEXT:     Name: .rela.text (17)
+# CHECK-NEXT:     Type: SHT_RELA (0x4)
+# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Address: 0x0
+# CHECK-NEXT:     Offset: 0x180
+# CHECK-NEXT:     Size: 36
+# CHECK-NEXT:     Link: 4
+# CHECK-NEXT:     Info: 1
+# CHECK-NEXT:     AddressAlignment: 4
+# CHECK-NEXT:     EntrySize: 12
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Section {
+# CHECK-NEXT:     Index: 4
+# CHECK-NEXT:     Name: .symtab (28)
+# CHECK:        }
+# CHECK-NEXT:   Section {
+# CHECK-NEXT:     Index: 5
+# CHECK-NEXT:     Name: .strtab (36)
+# CHECK:        }
+# CHECK:        Relocations [
+# CHECK-NEXT:     Section (2) .rel.text {
+# CHECK-NEXT:       0x1 R_MIPS_32 glob1 0x0
+# CHECK-NEXT:       0x1 R_MIPS_CALL16 glob2 0x0
+# CHECK-NEXT:       0x2 R_MIPS_LO16 loc1 0x0
+# CHECK-NEXT:     }
+# CHECK-NEXT:     Section (3) .rela.text {
+# CHECK-NEXT:       0x1 R_MIPS_32 glob1 0x1
+# CHECK-NEXT:       0x1 R_MIPS_CALL16 glob2 0x2
+# CHECK-NEXT:       0x2 R_MIPS_LO16 loc1 0x3
+# CHECK-NEXT:     }
+# CHECK-NEXT:   ]