powerpc CI does not run `assert_instr` tests on powerpc, and powerpc64 (big endian)

[gnzlbg]

The powerpc CI does not run assert_instr tests on powerpc and powerpc64 (bid endian). On powerpc qemu segfaults. The currently selected CPU is Vger and should support altivec, so this might be due to a bug in qemu. The powerpc64 (big endian) build bots never finish: maybe we aren't selecting the appropriate CPU for these tests to run?