[System.Core/Android] Android's libc.so doesn't export getpagesize()

Partially fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=12739

Android's libc.so doesn't export getpagesize(3), resulting in an
EntryPointNotFoundException:

	System.EntryPointNotFoundException: getpagesize
	  at (wrapper managed-to-native) System.IO.MemoryMappedFiles.MemoryMapImpl.getpagesize ()
	  at System.IO.MemoryMappedFiles.MemoryMapImpl.Map (int,long,long&,System.IO.MemoryMappedFiles.MemoryMappedFileAccess,intptr&,int&)
	  at System.IO.MemoryMappedFiles.MemoryMappedViewAccessor.Create (long,long,System.IO.MemoryMappedFiles.MemoryMappedFileAccess)
	  at System.IO.MemoryMappedFiles.MemoryMappedViewAccessor..ctor (int,long,long,System.IO.MemoryMappedFiles.MemoryMappedFileAccess)
	  at System.IO.MemoryMappedFiles.MemoryMappedFile.CreateViewAccessor (long,long,System.IO.MemoryMappedFiles.MemoryMappedFileAccess)

Android does provide getpagesize(3); it's just an inline function.

The full fix will require two parts:

 1. Update MemoryMapImpl to P/Invoke
    libmonodroid.so!monodroid_getpagesize()
 2. Add a libmonodroid.so!monodroid_getpagesize() export.

This commit implements (1).