ppc32_exec_DIVW can produce the wrong result in stable

[flaviojs]

The issue is already fixed in unstable.

The registers of this operation are supposed to be interpreted as signed numbers.
The temporary variables a and b have an unsigned type, which can produce a bad result with negative registers.
Examples:

a	b	unsigned	signed
-1	2	((m_uint32_t)-1) / 2 = 2147483647	-1 / 2 = 0
-1	-2	((m_uint32_t)-1) / ((m_uint32_t)-2) = 0	-1 / -2 = 2
1	-1	1 / ((m_uint32_t)-1) = 0	1 / -1 = -1

Discovered while converting to rust.