aarch64: add __ifunc_hwcap function to be used in ifunc resolvers

aarch64: add __ifunc_hwcap function to be used in ifunc resolvers

Add a new helper function __ifunc_hwcap() as a portable way to
access HWCAP elements via the parameter(s) passed to an ifunc
resolver checking the _IFUNC_ARG_HWCAP bit in the first parameter
and size of the buffer in the second parameter.

Note that 0 is returned when the requested element is not available
or does not correspond to a valid AT_HWCAP{,2,...} value.

Also add relevant tests.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>