ARM: Fix register constraints in run_in_exception_handler()

For asm() statements, clobbers are disjoint from inputs and outputs.

Therefore, run_in_exception_handler() specifying an input and clobbering
BUG_FN_REG forces the compiler to set up fn in the wrong regsiter, just to
have the code block move it into the correct register.

Instead, use `register asm()` to tell the compiler which register fn really
wants to be in.  This results in better code generation.

Bloat-o-meter reports:

  ARM64:
    Function                                     old     new   delta
    dump_registers                               356     348      -8

  ARM32:
    ns16550_poll                                  52      48      -4
    dump_registers                               432     428      -4

The other instruction dropped in ARM64's dump_registers() is an alignment nop.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>

xen/arch/arm/include/asm/bug.h

@@ -59,15 +59,15 @@ struct bug_frame {
  * be called function in a fixed register.
  */
 #define  run_in_exception_handler(fn) do {                                  \
-    asm ("mov " __stringify(BUG_FN_REG) ", %0\n"                            \
-         "1:"BUG_INSTR"\n"                                                  \
+    register unsigned long _fn asm (STR(BUG_FN_REG)) = (unsigned long)(fn); \
+    asm ("1:"BUG_INSTR"\n"                                                  \
          ".pushsection .bug_frames." __stringify(BUGFRAME_run_fn) ","       \
          "             \"a\", %%progbits\n"                                 \
          "2:\n"                                                             \
          ".p2align 2\n"                                                     \
          ".long (1b - 2b)\n"                                                \
          ".long 0, 0, 0\n"                                                  \
-         ".popsection" :: "r" (fn) : __stringify(BUG_FN_REG) );             \
+         ".popsection" :: "r" (_fn) );                                      \
 } while (0)
 
 #define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 0, "")