Use mostly the same registers for consistency

The exception to this is %eax is used not %edi on x86 because its
caller-save

src/arch/x86.rs

@@ -81,7 +81,7 @@ pub unsafe fn swap(arg: usize, old_sp: &mut StackPointer, new_sp: &StackPointer)
       # Remember stack pointer of the old context, in case %rdx==%rsi.
       movl    %esp, %ebx
       # Load stack pointer of the new context.
-      movl    (%edi), %esp
+      movl    (%edx), %esp
       # Save stack pointer of the old context.
       movl    %ebx, (%esi)
 
@@ -95,7 +95,7 @@ pub unsafe fn swap(arg: usize, old_sp: &mut StackPointer, new_sp: &StackPointer)
     : "={eax}" (ret)
     : "{eax}" (arg)
       "{esi}" (old_sp)
-      "{edi}" (new_sp)
+      "{edx}" (new_sp)
     : "eax",  "ebx",  "ecx",  "edx",  "esi",  "edi", //"ebp",  "esp",
       "mmx0", "mmx1", "mmx2", "mmx3", "mmx4", "mmx5", "mmx6", "mmx7",
       "xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5", "xmm6", "xmm7",

src/arch/x86_64.rs

@@ -58,17 +58,17 @@ pub unsafe fn swap(arg: usize, old_sp: &mut StackPointer, new_sp: &StackPointer)
 
     1:
       # Remember stack pointer of the old context, in case %rdx==%rsi.
-      movq    %rsp, %rax
+      movq    %rsp, %rbx
       # Load stack pointer of the new context.
       movq    (%rdx), %rsp
       # Save stack pointer of the old context.
-      movq    %rax, (%rsi)
+      movq    %rbx, (%rsi)
 
       # Pop instruction pointer of the new context (placed onto stack by
       # the call above) and jump there; don't use `ret` to avoid return
       # address mispredictions (~8ns on Ivy Bridge).
-      popq    %rax
-      jmpq    *%rax
+      popq    %rbx
+      jmpq    *%rbx
     2:
     "#
     : "={rdi}" (ret)