s2e-arm: bugfix: pass message pointer in r0 for s2earm calls

The pointer to the message for the s2e_{message,warning} calls was not
set in the 'r0' register.

Setting a register as clobbered doesn't guarantee that it will be used by the compiler to set the address of a parameter in it.

I think the same fix should be done for s2e.h (the header for x86).