ART: Relax GetInstructionSetFromString

Do not abort on an unknown instruction set string. Instead return
kNone and let the caller handle this.

Also simplify the patchoat tool to use this.

Bug: 17136416

(cherry picked from commit aabbb2066a715b3fd8e752291f74c6d77b970450)

Change-Id: I24131914bcf91c04ae93179bf809a2907f1f2b7a
diff --git a/runtime/instruction_set_test.cc b/runtime/instruction_set_test.cc
index ece3238..ac17c4f 100644
--- a/runtime/instruction_set_test.cc
+++ b/runtime/instruction_set_test.cc
@@ -29,6 +29,7 @@
   EXPECT_EQ(kX86_64, GetInstructionSetFromString("x86_64"));
   EXPECT_EQ(kMips, GetInstructionSetFromString("mips"));
   EXPECT_EQ(kNone, GetInstructionSetFromString("none"));
+  EXPECT_EQ(kNone, GetInstructionSetFromString("random-string"));
 }
 
 TEST_F(InstructionSetTest, GetInstructionSetString) {