[yaml2macho] Removing asserts in favor of explicit yaml parse error

32-bit Mach headers don't have reserved fields. When generating the
mapping for 32-bit headers leaving off the reserved field will result in
parse errors if the field is present in the yaml.

Added a CHECK-NOT line to ensure that mach_header.yaml isn't adding a
reserved field, and a test to ensure that the parser error gets hit with
32-bit headers.

llvm-svn: 273623