Disable powf/log2?f/exp2?f optimization for single-precision Arm FPU

New optimized powf, logf, log2f, expf and exp2f yield worse performance
on Arm targets with only single precision instructions because the
double precision arithmetic is then implemented via softfloat routines.
This patch uses the old implementation when double precision
instructions are not available on Arm targets.

Testing: Built newlib with GCC's rmprofile Arm multilibs and compared
before/after -> only the above functions are changed and calls to them
(name change from logf to __ieee754_logf and similar). Testing the
changed function on a panel of values yields the same result before the
original patches to improve them and after this one. Double checking the
performance by looping the same panel of values being tested on Arm
Cortex-M4 does show the performance regression is fixed.

newlib/libc/include/machine/ieeefp.h

@@ -78,7 +78,9 @@
 # else
 #  define __IEEE_BIG_ENDIAN
 # endif
-# define __OBSOLETE_MATH_DEFAULT 0
+# if __ARM_FP & 0x8
+#  define __OBSOLETE_MATH_DEFAULT 0
+# endif
 #else
 # define __IEEE_BIG_ENDIAN
 # ifdef __ARMEL__