x86_64: Add UD2 instruction and test.

This instruction is useful to guard unreachable code that we have to
generate for some rason, but we should never execute.

Bug: 71781600
Test: $ANDROID_HOST_OUT/nativetest64/art_compiler_host_tests/art_compiler_host_tests --gtest_filter='*AssemblerX86_64Test.Ud2*'
Change-Id: Ibd106c0091bd3fd958ab34f0b6d90382773aad79
3 files changed