libvpx: do not hardcode -mfloat-api=softfp for ARM builds

| Configuring for target 'armv7-linux-gcc'
|   enabling armv7
|   enabling armv6
|   enabling armv5te
|   enabling fast_unaligned
| Unable to invoke compiler: arm-oe-linux-gnueabi-gcc  -march=armv7-a -marm -mthumb-interwork -mfloat-abi=hard -mfpu=neon --sysroot=/home/hrw/HDD/devel/canonical/aarch64/openembedded/build/tmp-eglibc/sysroots/genericarmv7a -O2 -pipe -g -feliminate-unused-debug-types -fPIC -march=armv7-a -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -O3 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Wdeclaration-after-statement -Wdisabled-optimization -Wpointer-arith -Wtype-limits -Wcast-qual -Wno-unused
|
| Configuration failed. This could reflect a misconfiguration of your
| toolchains, improper options selected, or another problem. If you
| don't see any useful error messages above, the next step is to look
| at the configure error log file (config.err) to determine what
| configure was trying to do when it died.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>