save AT_HWCAP from auxv for subsequent use in machine-specific code

save AT_HWCAP from auxv for subsequent use in machine-specific code

it's expected that this will be needed/useful only in asm, so I've
given it its own symbol that can be addressed in pc-relative ways from
asm rather than adding a field in the __libc structure which would
require hard-coding the offset wherever it's used.