[ARM] Refactor strlen.c #if nesting.

This patch flattens the condition code selection used in strlen in an
attempt to make the guarding condition for each alternative
implementation clearer and to structure the logic in a manner that
makes it easier to maintain complementary logic between the
alternative 'C' and assembler implementations.

Tested by building newlib and comparing libc.a binaries before and
after for all permutations of:

  Architectures:
    armv4 armv4t armv5 armv5t armv5te armv6 armv6j armv6k
    armv6z armv6kz armv6t2 armv6-m armv6s-m armv7 armv7-a
    armv7ve armv7-r armv7-m armv7e-m armv8-a iwmmxt iwmmxt2

  ISAs:
    thumb arm

  Optimization Levels:
    Os O2

  Excluding:
    armv6s-m -mthumb
    armv6-m -mthumb

    armv6zk -mthumb
    armv6z -mthumb
    armv6k -mthumb
    armv6j -mthumb

newlib/ChangeLog

@@ -1,3 +1,7 @@
+2015-11-13  Marcus Shawcroft  <marcus.shawcroft@arm.com>
+
+	* libc/machine/arm/strlen.c: Refactor nested #if.
+
 2015-11-12  Anton Kolesov  <Anton.Kolesov@synopsys.com>
 
 	* configure.host: Add ARC support.

newlib/libc/machine/arm/strlen.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008 ARM Ltd
+ * Copyright (c) 2008-2015 ARM Ltd
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -31,32 +31,12 @@
 #include <string.h>
 #include <limits.h>
 
-#if defined (__OPTIMIZE_SIZE__) || defined (PREFER_SIZE_OVER_SPEED) || \
+#if defined __OPTIMIZE_SIZE__ || defined PREFER_SIZE_OVER_SPEED
-  (defined (__thumb__) && !defined (__thumb2__))
+#if defined __thumb__ && !defined __thumb2__
-
-# if !defined (PREFER_SIZE_OVER_SPEED) && !defined (__OPTIMIZE_SIZE__)
-/* Thumb1 only variant.
-   If speed is preferred, the strlen() function in ../../string/strlen.c
-   will be used.
-
-   Leave this field blank.  So the strlen() is not defined, and this will
-   automatically pull in the default C definition of strlen() from
-   ../../string/strlen.c.  No need to include this file explicitely.
-   The lib_a-strlen.o will not be generated, so it won't replace the default
-   lib_a-strlen.o which is generated by ../../string/strlen.c.  See the
-   commands in configure.in and Makefile.am for more details.
-
-   However, if we need to rewrite this function to be more efficient,
-   we can add the corresponding assembly code into this field and change
-   the commands in configure.in and Makefile.am to allow the corresponding
-   lib_a-strlen.o to be generated.
-*/
-# else
 size_t
 strlen (const char* str)
 {
   int scratch;
-#if defined (__thumb__) && !defined (__thumb2__)
   size_t len;
   asm ("mov	%0, #0\n"
        "1:\n\t"
@@ -66,7 +46,13 @@ strlen (const char* str)
        "bne	1b"
        : "=&r" (len), "=&r" (scratch) : "r" (str) : "memory", "cc");
   return len - 1;
+}
+
 #else
+size_t
+strlen (const char* str)
+{
+  int scratch;
   const char* end;
   asm ("1:\n\t"
        "ldrb	%1, [%0], #1\n\t"
@@ -74,13 +60,17 @@ strlen (const char* str)
        "bne	1b"
        : "=&r" (end), "=&r" (scratch) : "0" (str) : "memory", "cc");
   return end - str - 1;
-#endif
 }
 #endif
+
+#else /* defined __OPTIMIZE_SIZE__ || defined PREFER_SIZE_OVER_SPEED */
+#if defined __thumb__ && ! defined __thumb2__
+  /* Implemented in ../../string/strlen.c.  */
+
+#elif defined _ISA_ARM_7 || defined __ARM_ARCH_6T2__
+  /* Implemented in strlen-armv7.S.  */
+
 #else
-
-#if !(defined(_ISA_ARM_7) || defined(__ARM_ARCH_6T2__))
-
 size_t __attribute__((naked))
 strlen (const char* str)
 {