Mimic 8-bit overflow manually in SCBRR2_write

[vkedwardli]

Fixes MUSHIKING cannot insert card with macOS arm64. (Issue reported from Discord)

Details:
In macOS arm64, the SCIFSerialPort::schedCallback would stop calling after the Now Loading... screen.

SCIFSerialPort::schedCallback
SCIFSerialPort::rxSched()
return scif.frameSize * scif.cyclesPerBit (10, 2560)
SH4 SCIF: Frame size 10 cycles/bit 2560 (78125 bauds) pipe 0x600002b78460, SCIF_SCBRR2 = 19, SCIF_SCSMR2.CKS = 0
SCIFSerialPort::SCBRR2_write: 4294967202
SH4 SCIF: Frame size 10 cycles/bit 0 (0 bauds) pipe 0x600002b78460, SCIF_SCBRR2 = 4294967202, SCIF_SCSMR2.CKS = 0
SCIFSerialPort::schedCallback
SCIFSerialPort::rxSched()
return scif.frameSize * scif.cyclesPerBit (10, 0)

This happens when SCIFSerialPort::SCBRR2_write is trying to write a -94, overflowing to become 4294967202, making cyclesPerBit become 0.
(But on the debug build, it is writing 162, the 8-bit overflow value for -94)

Since static_cast<signed char>(data) or static_cast<u8>(static_cast<signed char>(data)> does not work, so I just add 256 directly to mimic the 8-bit overflow

Question:

• Should I add ifdef directive so the new code only runs on macOS arm64?
• Is there a better 8-bit overflow handling?

[vkedwardli]

It is writing FFFFFFA2 (4294967202) which should be masked as 000000A2 (162) for u8