ANDROID: 16K: Align vdso mapping size to a 16kB multiple

The x86 emulated page size is controlled by runtime kernel param
page_shift=*, to avoid needing a separate x86 16k kernel build.

The vdso is a special VMA inserted in the address space of
every process. It is not created by do_mmap(), and therefore
needs to be make 16kB compatible separately.

Align mapping size by the max page size, since this must be
known at build time when converting the vdso shared library
to a C array.

The target page size is not known at build time so use the
larger, more portable, alignment/size.

x86 builds will have a extended vdso layout. This is an
anonymous mapping so it is not be affected by readahead.
The cost on 4kB x86 device is extra VA space (at most 12kB).

Bug: 356729201
Change-Id: I0bbc181c3542183df340eaf6cf55de959541603f
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>