tcg/tci: Fix TCG_REG_R4 misusage

tcg/tci: Fix TCG_REG_R4 misusage

This was removed from tcg_target_reg_alloc_order and
tcg_target_call_iarg_regs on the assumption that it
was the stack.  This was incorrectly copied from i386.
For tci, the stack is R15.

By adding R4 back to tcg_target_call_iarg_regs, adjust the other
entries so that 6 (or 12) entries are still present in the array,
and adjust the numbers in the interpreter.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>