[yaml2obj] - Don't crash when `FileHeader` declares an empty `Flags` key in specific situations.

We currently call the `llvm_unreachable` for the following YAML:

```
--- !ELF
FileHeader:
  Class:   ELFCLASS32
  Data:    ELFDATA2LSB
  Type:    ET_REL
  Machine: EM_NONE
  Flags:   [ ]
```

it happens because the `Flags` key is present, though `EM_NONE` is a
machine type that has no known `EF_*` values and we call `llvm_unreachable` by mistake.

Differential revision: https://reviews.llvm.org/D86138

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -434,10 +434,8 @@ void ScalarBitSetTraits<ELFYAML::ELF_EF>::bitset(IO &IO,
     BCase(EF_AMDGPU_XNACK);
     BCase(EF_AMDGPU_SRAM_ECC);
     break;
-  case ELF::EM_X86_64:
-    break;
   default:
-    llvm_unreachable("Unsupported architecture");
+    break;
   }
 #undef BCase
 #undef BCaseMask

llvm/test/tools/yaml2obj/ELF/eflags.yaml

@@ -0,0 +1,16 @@
+## Check how the 'Flags' key can be used to encode e_flags field values.
+
+## Check we are able to produce no flags for EM_NONE. EM_NONE is an arbitrary
+## e_machine type that has no EF_* values defined for it.
+# RUN: yaml2obj %s -o %t-no-flags
+# RUN: llvm-readelf --file-headers %t-no-flags | FileCheck %s --check-prefix=NOFLAGS
+
+# NOFLAGS: Flags: 0x0{{$}}
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_NONE
+  Flags:   [ ]