[x64] support syscalls in x86 mode

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on June 21, 2012 15:20:37_

this was PR 284029

split from issue #49 for windows: if going to let wow64 layer run native, want x64 DR rolling
its own 64-bit syscalls but need wow64 do_syscall for app. may be simpler
to control wow64 layer: then there are no 32-bit syscalls and the wow64 ind call is just that: an ind call.

for linux there are 32-bit syscalls (kernel does the marshalling) so we'd
need to support both there

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original notes:

split from PR 253431

for now we only support WOW64 and assume there are no syscalls in x86 mode

note that we would need to update not only emit_shared_syscall and emit_do_syscall but
also emit_detach_callback_code to fully support x86 syscalls on Windows

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=821_