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>

xen/Rules.mk

@@ -76,13 +76,6 @@ endif
 
 AFLAGS-y                += -D__ASSEMBLY__
 
-# Older clang's built-in assembler doesn't understand .skip with labels:
-# https://bugs.llvm.org/show_bug.cgi?id=27369
-ifeq ($(clang),y)
-$(call as-option-add,CFLAGS,CC,".L0:\n.L1:\n.skip (.L1 - .L0)",,\
-                     -no-integrated-as)
-endif
-
 ALL_OBJS := $(ALL_OBJS-y)
 
 # Get gcc to generate the dependencies for us.

xen/arch/x86/Rules.mk

@@ -74,6 +74,11 @@ ifeq ($(clang),y)
 # Note: Any test which adds -no-integrated-as will cause subsequent tests to
 # succeed, and not trigger further additions.
 
+# Older clang's built-in assembler doesn't understand .skip with labels:
+# https://bugs.llvm.org/show_bug.cgi?id=27369
+$(call as-option-add,CFLAGS,CC,".L0: .L1: .skip (.L1 - .L0)",,\
+                     -no-integrated-as)
+
 # Check whether clang asm()-s support .include.
 $(call as-option-add,CFLAGS,CC,".include \"asm/indirect_thunk_asm.h\"",,\
                      -no-integrated-as)