package/uhd: fix build with ARM without NEON

package/uhd: fix build with ARM without NEON

UHD has two issues with ARM:
1/ NEON_SIMD_ENABLE is always enabled when architecture is ARM. But NEON isn't
   necessarily support in the CPU
2/ when NEON is supported -mfpu=neon must be passed to the compiler (not
   automatically done by UHD)

Fixes:
- http://autobuild.buildroot.net/results/2519bd075b8c021b440677297b32f16b1c3cc3dd/
- http://autobuild.buildroot.net/results/99d9979be4f37c42390c87b1afd4cd33bac07f5e/
- http://autobuild.buildroot.net/results/cabc7a87ee869d6bac7d092c1a4fcfcdac22a96e/

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>