x86: fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"

x86: fix build following c/s 623c720f "x86: use CLFLUSHOPT when available"

CentOS 7 gets into trouble when compiling Xen citing:

  flushtlb.c: Assembler messages:
  flushtlb.c:149: Error: value of 256 too large for field of 1 bytes at 1

The line number is wrong, and the error message not helpful.  It turns out
that the intermediate generated assembly was

  # 139 "arch/x86/flushtlb.c" 1
      661:
      rex clflush (%r15)
  662:
  .pushsection .altinstructions,"a"

and it was having trouble combining the explicit REX prefix with the REX.B
required for the use of %r15.

Follow what Linux does and use a redundant %ds prefix instead, for a final
generated instruction of `3e 41 0f ae 3f`

While modifying this line, fix the indentation which was out by one space.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Doug Goldstein <cardoe@cardoe.com>
Introduce and use NOP_DS_PREFIX.

Also the above description of the issue is slightly wrong: We're not
suffering from some gas versions not being able to combine multiple REX
prefixes, but from the replacement instruction, when requiring a REX
prefix in order to express the memory operand, becoming one byte longer
than the original one, triggering the respective build time safety
check.

Signed-off-by: Jan Beulich <jbeulich@suse.com>