provide getauxval(AT_SECURE) even if it is missing from the aux vector

this could happen on 2.4-series linux kernels that predate AT_SECURE
and possibly on other kernels that are emulating the linux syscall API
but not providing AT_SECURE in the aux vector at startup.

in principle applications should be checking errno anyway, but this
does not really work. to be secure, the caller would have to treat
ENOENT (indeterminate result) as possibly-suid and thereby disable
functionality in the typical non-suid usage case. and since glibc only
runs on kernels that provide AT_SECURE, applications written to the
glibc getauxval API might simply assume it succeeds.

add getauxval function

in a sense this implementation is incomplete since it doesn't provide
the HWCAP_* macros for use with AT_HWCAP, which is perhaps the most
important intended usage case for getauxval. they will be added at a
later time.