Pistachio: tell the compiler about 'edi' changes

Syscall binding functions which use '__L4_INDIRECT_CALL' need to tell the
compiler that the 'edi' register gets modified. Since 'edi' is an input
operand and input operands may not get added to the clobber list, this
patch defines 'edi' also as an output operand instead.

Fixes #834.