Update the generic 32 bit arm linux cpu feature detection to support 64 bit kernels

On 64 bit kernels, /proc/cpuinfo doesn't list the same old features
as on 32 bit kernels, since most of them are mandatory on 64 bit arm
systems, see [1] for details.

If running a 32 bit arm binary on such a kernel, we need to detect
the features slightly differently, either by using other names
listed in the Features field on these devices, by checking the
"CPU architecture" field and always enabling these if the architecture
is >= 8, or by parsing /proc/self/auxv or using the getauxval function.

[1] http://marc.info/?l=linux-arm-kernel&m=139087240101974
Merge pull request #1047 from mstorsjo/cpufeatures-armv8

Update the generic 32 bit arm linux cpu feature detection to support 64 bit kernels