x86/time: fix scale_delta() inline assembly

The way it was coded, it clobbered %rdx without telling the compiler.
This generally didn't cause any problems except when there are two back
to back invocations (as in plt_overflow()), as in that case the
compiler may validly assume that it can re-use for the second instance
the value loaded into %rdx before the first one.

Once at it, also properly relax the second operand of "mul" (there's no
need for it to be in %rdx, or a register at all), and switch away from
using explicit register names in the instruction operands.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -127,8 +127,9 @@ static inline u64 scale_delta(u64 delta,
         delta <<= scale->shift;
 
     asm (
-        "mul %%rdx ; shrd $32,%%rdx,%%rax"
-        : "=a" (product) : "0" (delta), "d" ((u64)scale->mul_frac) );
+        "mul %2 ; shrd $32,%1,%0"
+        : "=a" (product), "=d" (delta)
+        : "rm" (delta), "0" ((u64)scale->mul_frac) );
 
     return product;
 }