Inconsistency with real CpuSet

[aarant]

The behavior of the BIOS function/software interrupt CpuSet in mGBA differs from that of a real GBA BIOS.

In mGBA, when the software interrupt returns, r0 and r1 will both have been incremented by the number of bytes copied.

Using a real BIOS, when the software interrupt returns, r0 and r1 are unchanged.

Example code to reproduce:

.text
.syntax unified
.thumb
.align 1
  adr r0, data
  ldr r1, dest
  movs r2, #3 @ 3 halfwords
  swi 11
  @ r0 and r1 should be unchanged
.align 2
dest: .4byte 0x02000000
data: .byte 1 2 3 4 5 6

sha256 of BIOS used: fd2547724b505f487e6dcb29ec2ecff3af35a841a77ab2e85fd87350abd36570

Build: 0.7.2
OS: Fedora 29 64-bit

I've reproduced this issue in Pokemon Emerald, where my code was working in mGBA but failing in Bizhawk in my TAS (BizHawk requires using a real BIOS). The code makes a lot of use of CpuSet.