vecobj.t exposes Moonjit bugs on PPC64le

[elliottslaughter]

The vecobj.t test appears to expose multiple bugs in the Moonjit support for the C calling convention. I'm reasonably certain that these are Moonjit bugs rather than Terra bugs because we have extremely comprehensive tests against Clang, which Terra passes with flying colors.

1. Callbacks: the printfloat callback into Lua ({float}->{}) results in a segfault.
2. Calling a C/Terra function with a struct (containing arrays) results in garbage.

I'm going to patch the test to work around this since there isn't much we can do about Moonjit limitations.

[elliottslaughter]

The second of these two issues (disagreement in the calling convention for arrays) applies to AArch64 as well. Since I'm pretty sure we're doing this correctly (thanks to cconv_more.t and cconv_array.t) I'm going to apply the workaround here as well.