ART: Show invalid oat header cause in error message

Include the IsValid failure cause in the error message of opening
an oat file.

Make oat magic and version constexpr so we can have static_asserts
over the lengths.

Bug: 17187621
Change-Id: I8cef1c6cc9cb2faa052167a3437e81b4c2cbefa7
Merge "ART: Show invalid oat header cause in error message"
ART: Break up x86 disassembler main function

The function leads to large stack frames with Clang. Break out
some parts and use four char* variables for opcode.

Bug: 18733806
Change-Id: I8bf6da6c763175d7081c4231fa5d3b6809316220
Merge "ART: Break up x86 disassembler main function"