[nolibc] Move GetPageSize to the individual platforms.

GetPageSize wraps sysconf(_SC_PAGESIZE) on POSIX platforms, but
sysconf resides in libc.  To make this libc-independent on Linux,
move the wrapper to sanitizer_mac.cc and return the Linux-specific
constant EXEC_PAGESIZE in the sanitizer_linux.cc implementation.

llvm-svn: 182303