x86/paravirt: Use %rip-relative addressing in hook calls

While using a plain (constant) address works, its use needlessly invokes
a SIB addressing mode, making every call site one byte larger than
necessary:

  ff 14 25 98 89 42 82    call   *0xffffffff82428998

Instead of using an "i" constraint with address-of operator and a 'c'
operand modifier, simply use an ordinary "m" constraint, which the
64-bit compiler will translate to %rip-relative addressing:

  ff 15 62 fb d2 00       call   *0xd2fb62(%rip)	# ffffffff82428998 <pv_ops+0x18>

This way the compiler is also told the truth about operand usage - the
memory location gets actually read, after all.

32-bit code generation is unaffected by the change.

  [ bp: Remove "we", add examples. ]

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/b8192e8a-13ef-6ac6-6364-8ba58992cd1d@suse.com