Update architecture status to align more with official kernel documentation

1. Upgrade s390 and riscv to supported in limited configurations. s390
   has had fantastic support from IBM and Red Hat across the Linux
   kernel, LLVM, and QEMU. It might even be worth bumping it to well
   maintained.

2. Add link to m68k's backend support in LLVM.

3. Update csky's link as it has recently changed.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

readme.md

@@ -37,24 +37,24 @@ and are relatively well supported and tested:
 
 The following architectures are targetable from both LLVM and the Linux kernel
 and are supported and tested in limited configurations:
-* powerpc
 * mips
+* powerpc
+* riscv
+* s390
 
 The following architectures are targetable from both LLVM and the Linux kernel
 but are untested or currently broken:
 * arc
 * hexagon
-* riscv
-* s390
 * sparc
 
 The following architectures are targetable from the Linux kernel but not LLVM:
 * alpha
 * c6x
-* csky ([WIP](https://reviews.llvm.org/D86269))
+* csky ([WIP](https://reviews.llvm.org/D93798))
 * h8300
 * ia64
-* m68k
+* m68k ([WIP](https://reviews.llvm.org/D95315))
 * microblaze
 * nds32
 * nios2