[ELF] Properly reject unsupported e_machine

Fix an incorrect llvm_unreachable.

Author: MaskRay

lld/ELF/Target.cpp

@@ -89,8 +89,9 @@ TargetInfo *elf::getTarget() {
     return getSPARCV9TargetInfo();
   case EM_X86_64:
     return getX86_64TargetInfo();
+  default:
+    fatal("unsupported e_machine value: " + Twine(config->emachine));
   }
-  llvm_unreachable("unknown target machine");
 }
 
 ErrorPlace elf::getErrorPlace(const uint8_t *loc) {

lld/test/ELF/unsupported-emachine.test

@@ -0,0 +1,11 @@
+# RUN: yaml2obj %s -o %t.o
+# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
+
+# CHECK: error: unsupported e_machine value: 9
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_S370