[libc] Move the implementation of mmap and munmap into a linux specific area.

This allows us to get rid of the PAGE_SIZE macro and use EXEC_PAGESIZE
from linux/param.h.

Few other points about this change:
1. The linux syscall functions have been moved into a linux specific area
instead of src/unistd/syscall.h. The Linux syscall function from unistd.h
is a public vararg function. What we have currently are linux speciif internal
overloaded C++ functions. So, moving them to a Linux only area is more
meaningful.
2. The implementations of mmap and munmap are now in a 'linux' directory
within src/sys/mman. The idea here is that platform specific
implementations will live in a platform specific subdirectories like these.
Infrastructure common to a platform will live in the platform's config
directory. For example, the linux syscall implementations live in
config/linux.

Reviewers: abrachet

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D73302