[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.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182303 91177308-0d34-0410-b5e6-96231b3b80d8