ARM: clean up the memset64() C wrapper

The current logic to split the 64-bit argument into its 32-bit halves is
byte-order specific and a bit clunky.  Use a union instead which is
easier to read and works in all cases.

GCC still generates the same machine code.

While at it, rename the arguments of the __memset64() prototype to
actually reflect their semantics.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/include/asm/string.h

diff --git a/arch/arm/include/asm/string.h b/arch/arm/include/asm/string.h
index c35250c..96fc6cf 100644
--- a/arch/arm/include/asm/string.h
+++ b/arch/arm/include/asm/string.h
@@ -39,13 +39,17 @@ static inline void *memset32(uint32_t *p, uint32_t v, __kernel_size_t n)
 }
 
 #define __HAVE_ARCH_MEMSET64
-extern void *__memset64(uint64_t *, uint32_t low, __kernel_size_t, uint32_t hi);
+extern void *__memset64(uint64_t *, uint32_t first, __kernel_size_t, uint32_t second);
 static inline void *memset64(uint64_t *p, uint64_t v, __kernel_size_t n)
 {
-	if (IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN))
-		return __memset64(p, v, n * 8, v >> 32);
-	else
-		return __memset64(p, v >> 32, n * 8, v);
+	union {
+		uint64_t val;
+		struct {
+			uint32_t first, second;
+		};
+	} word = { .val = v };
+
+	return __memset64(p, word.first, n * 8, word.second);
 }
 
 /*