[libc] provide compat sysconf values

[nickdesaulniers]

As discovered in #85971, we provide a definition of _SC_PAGESIZE whose value does not match glibc's (or bionic's).

POSIX defines that these macro definitions exist, but not what value they have.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html

This can lead to compatibility bugs. We may need to provide definitions for these.