AMDGPU: Add and set AMDGPU-specific e_flags

Differential Revision: https://reviews.llvm.org/D38556

llvm-svn: 314987
diff --git a/llvm/test/tools/llvm-readobj/amdgpu-elf-definitions.test b/llvm/test/tools/llvm-readobj/amdgpu-elf-definitions.test
index e52f987..9b077ff 100644
--- a/llvm/test/tools/llvm-readobj/amdgpu-elf-definitions.test
+++ b/llvm/test/tools/llvm-readobj/amdgpu-elf-definitions.test
@@ -1,7 +1,7 @@
 RUN: llvm-readobj -file-headers -program-headers -sections -symbols %p/Inputs/trivial.obj.elf-amdhsa-gfx803 | FileCheck %s
 
 CHECK: Format: ELF64-amdgpu
-CHECK: Arch: amdgcn
+CHECK: Arch: unknown
 CHECK: ElfHeader {
 CHECK:   Ident {
 CHECK:     OS/ABI: AMDGPU_HSA (0x40)