linker: Purge block allocator memory when possible

If all allocated memory from a block allocator is freed, it is a good
opportunity to purge all the pages allocated to reduce lingering dirty
pages.

Memory saving varies with the platform and what processes are running.
Measuring right after boot, this saves ~1.8MB on cuttelfish and ~1.3MB
on a 32-bit ARM device.

Bug: 112073665
Test: Boot and check memory usage with 'showmap'.
Change-Id: I53769e0ec9699f0b3645cdf281a2c0bbffb98676