start: Add clrrdi to align the stack pointer on powerpc64.

Author: alexrp

lib/std/start.zig

@@ -340,10 +340,10 @@ fn _start() callconv(.Naked) noreturn {
             ,
             .powerpc64, .powerpc64le =>
             // Setup the initial stack frame and clear the back chain pointer.
-            // TODO: Support powerpc64 (big endian) on ELFv2.
             \\ addis 2, 12, .TOC. - _start@ha
             \\ addi 2, 2, .TOC. - _start@l
             \\ mr 3, 1
+            \\ clrrdi 1, 1, 4
             \\ li 0, 0
             \\ stdu 0, -32(1)
             \\ mtlr 0