x86: move and fix clang .skip check

x86: move and fix clang .skip check

.skip is only used by x86 code, so place the clang .skip with labels
check in x86/Rules.mk instead of the top level Rules.mk. While there
also fix an issue with it by removing the '\n' which triggers the
following error:

<stdin>:1:31: error: missing terminating '"' character [-Werror,-Winvalid-pp-token]
void _(void) { asm volatile ( ".L0:
                              ^
<stdin>:1:31: error: expected string literal in 'asm'
<stdin>:3:18: error: missing terminating '"' character [-Werror,-Winvalid-pp-token]
.skip (.L1 - .L0)" ); }
                 ^
<stdin>:3:24: error: expected ')'
.skip (.L1 - .L0)" ); }
                       ^
<stdin>:1:29: note: to match this '('
void _(void) { asm volatile ( ".L0:
                            ^
<stdin>:3:24: error: expected '}'
.skip (.L1 - .L0)" ); }
                       ^
<stdin>:1:14: note: to match this '{'
void _(void) { asm volatile ( ".L0:
             ^
5 errors generated.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com> [On FreeBSD and Debian 9.5]
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>