Pre-allocate necessary heap space with boot image.

Previously we relied on a subsequent mmap to succeed
without prior memory reservation.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Test: m test-art-target-gtest
Test: testrunner.py --target --optimizing
Test: Repeat the above with ART_USE_READ_BARRIER=false.
Bug: 77856493
Change-Id: I1e433b1b6fecd0f908c7ea079f74361e27a78f0a