Add support to configure sysconf as part of app_config

[shaikshavali1]

In SGX-LKL, we have a mechanism to set sysconf values here: https://github.com/lsds/sgx-lkl/blob/529f6ca86d9937dfb31685c65f78588cf5ff39fa/src/main-oe/sgxlkl_run_oe.c#L694 Currently this is untrusted, and it should be extended to take values from the app_config.

One of the LTP test cases (setgroups03) fails because it uses sysconf(_SC_NGROUPS_MAX) but musl sets this to 32 by default: build_musl/host-musl/include/limits.h -- #define NGROUPS_MAX 32