Implement the BXNS v8M instruction, which is like BX but will do a
jump-and-switch-to-NonSecure if the branch target address has bit 0
clear.
This is the first piece of code which implements "switch to the
other security state", so the commit also includes the code to
switch the stack pointers around, which is the only complicated
part of switching security state.
BLXNS is more complicated than just "BXNS but set the link register",
so we leave it for a separate commit.
Backports commit fb602cb726b3ebdd01ef3b1732d74baf9fee7ec9 from qemu
We've currently got 18 architectures in QEMU, and thus 18 target-xxx
folders in the root folder of the QEMU source tree. More architectures
(e.g. RISC-V, AVR) are likely to be included soon, too, so the main
folder of the QEMU sources slowly gets quite overcrowded with the
target-xxx folders.
To disburden the main folder a little bit, let's move the target-xxx
folders into a dedicated target/ folder, so that target-xxx/ simply
becomes target/xxx/ instead.
Backports commit fcf5ef2ab52c621a4617ebbef36bf43b4003f4c0 from qemu
