mk: improve ARM NEON detection

mk: improve ARM NEON detection

The __ARM_NEON declares that the arm_neon.h is available
which is not always true for the __ARM_NEON_FP.

 $ arm-linux-gnueabi-gcc -dM -E - < /dev/null  | grep "_FP\|_NEON"
 #define __ARM_FP 12
 #define __ARM_NEON_FP 4
 #define __VFP_FP__ 1

 $ arm-linux-gnueabi-gcc -mfpu=neon -dM -E - < /dev/null  | grep "_FP\|_NEON"
 #define __ARM_FP 12
 #define __ARM_NEON_FP 4
 #define __ARM_NEON__ 1
 #define __VFP_FP__ 1
 #define __ARM_NEON 1

 $ aarch64-linux-gnu-gcc -dM -E - < /dev/null | grep "NEON\|FP"
 #define __FP_FAST_FMAF 1
 #define __ARM_NEON 1
 #define __FP_FAST_FMA 1

 $ aarch64-thunderx-linux-gnu-gcc -dM -E - < /dev/null |grep "NEON\|FP"
 #define __ARM_FP 12
 #define __ARM_NEON_FP 12
 #define __FP_FAST_FMAF 1
 #define __ARM_NEON 1
 #define __FP_FAST_FMA 1

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>