linker: improve "bad ELF magic" error.

Include what we actually saw.

Bug: http://b/79463184
Test: ran tests
Change-Id: Id5f8aa3ab94b411ea839b0a1ada9beda9053a10f
diff --git a/tests/dlext_test.cpp b/tests/dlext_test.cpp
index ec27932..1e3d9c8 100644
--- a/tests/dlext_test.cpp
+++ b/tests/dlext_test.cpp
@@ -173,7 +173,7 @@
   extinfo.library_fd_offset = 0;
   handle_ = android_dlopen_ext("libname_ignored", RTLD_NOW, &extinfo);
   ASSERT_TRUE(handle_ == nullptr);
-  ASSERT_EQ("dlopen failed: \"" + lib_path + "\" has bad ELF magic", dlerror());
+  ASSERT_EQ("dlopen failed: \"" + lib_path + "\" has bad ELF magic: 504b0304", dlerror());
 
   // Check if dlsym works after unsuccessful dlopen().
   // Supply non-exiting one to make linker visit every soinfo.