Replace enclave mmap with LKL mmap implementation

[prp]

Currently SGX-LKL has a separate mmap implementation, and does not use LKL's mmap() for heap allocations. A drawback of this design is that LKL is unaware of userspace allocations and maintains its own fragmented memory space.

Instead, we should remove the enclave mmap() implementation and have all allocations be handled by LKL.

Currently, replacing the enclave mmap allocator with the LKL one results in a segfault in the musl libc dynamic loader.