powerpc/reg: use ASM_FTR_IFSET() instead of opencoding fixup.

mftb() includes a feature fixup for CELL ppc.

Use ASM_FTR_IFSET() macro instead of opencoding the setup
of the fixup sections.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ac19713826fa55e9e7bfe3100c5a7b1712ab9526.1566999711.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/reg.h

@@ -1384,19 +1384,9 @@ static inline void msr_check_and_clear(unsigned long bits)
 #define mftb()		({unsigned long rval;				\
 			asm volatile(					\
 				"90:	mfspr %0, %2;\n"		\
+				ASM_FTR_IFSET(				\
 					"97:	cmpwi %0,0;\n"		\
-				"	beq- 90b;\n"			\
-				"99:\n"					\
-				".section __ftr_fixup,\"a\"\n"		\
-				".align 3\n"				\
-				"98:\n"					\
-				"	.8byte %1\n"			\
-				"	.8byte %1\n"			\
-				"	.8byte 97b-98b\n"		\
-				"	.8byte 99b-98b\n"		\
-				"	.8byte 0\n"			\
-				"	.8byte 0\n"			\
-				".previous"				\
+					"	beq- 90b;\n", "", %1)	\
 			: "=r" (rval) \
 			: "i" (CPU_FTR_CELL_TB_BUG), "i" (SPRN_TBRL) : "cr0"); \
 			rval;})