Darwin AArch64 calling conventions are wrong in NCG

It appears that all is not well in our treatment of Darwin AArch64 calling conventions. Specifically, consider the program in $3141:

$ _build/ghc-stage1 --info
...
 ,("target platform string","aarch64-apple-darwin")
 ,("target os","OSDarwin")
 ,("target arch","ArchAArch64")
 ,("target word size","8")
...
 ,("Project Git commit id","4ae0cde9cb1d3ca0d34efe049bda37d52c2bac10")
...

$ _build/ghc-stage1 -O0 -g3 -optc-g3 TestHs.hs TestC.c -o Main
$ ./Main
0x1111111111111111
0x2222222222222222
0x3333333333333333
4
5
6
0x7777777777777777
0x8888888888888888
0x9999999999999999
0xaaaaaaaaaaaaaaaa
0xbbbbbbbbbbbbbbbb
cccccccc
0x1dddddddd

$ _build/ghc-stage1 -O0 -g3 -optc-g3 TestHs.hs TestC.c -o Main -fllvm
$ ./Main
0x1111111111111111
0x2222222222222222
0x3333333333333333
4
5
6
0x7777777777777777
0x8888888888888888
0x9999999999999999
0xaaaaaaaaaaaaaaaa
0xbbbbbbbbbbbbbbbb
cccccccc
0xdddddddddddddddd

Look at the output from the last printf, which should print the value of the failed_doing argument (0xdddddddddddddddd) but actually prints 0x1dddddddd. Clearly this is nonsense.