handle execve of different-architecture child: 32 to 64, 64 to 32

From [email protected] on February 16, 2009 17:42:26

we'll need to decide from the parent which lib32 vs lib64 to use

xref the analogue on Windows: PR 254193: [x64] inject into different-architecture child: x64 to WOW64, WOW64 to x64

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=20