sysinfo: use libc getauxval instead of coding our own.

Avoids issues with opening /proc/self/auxv or relying on ABI details like auxv position in relation to environ (if a library with a constructor uses setenv, environ might have changed and not be the one passed by the kernel anymore).

Both musl and glibc implement the getauxval() function and define the AT_SECURE and AT_CLKTCK constants in their <elf.h> headers.