[WebAssembly] Define WebAssembly-specific relocation codes.

Currently WebAssembly has two kinds of relocations; data addresses and
function addresses. This adds ELF relocations for them, as well as an
MC symbol kind to indicate which type of relocation is needed.

llvm-svn: 257416
diff --git a/llvm/test/CodeGen/WebAssembly/unreachable.ll b/llvm/test/CodeGen/WebAssembly/unreachable.ll
index 2b03112..7b23bf3 100644
--- a/llvm/test/CodeGen/WebAssembly/unreachable.ll
+++ b/llvm/test/CodeGen/WebAssembly/unreachable.ll
@@ -12,7 +12,7 @@
 declare void @abort()
 
 ; CHECK-LABEL: f1:
-; CHECK: call abort
+; CHECK: call abort@FUNCTION{{$}}
 ; CHECK: unreachable
 define i32 @f1() {
   call void @abort()