Fix __cxa_atexit run-time regression with many registrations

Fix __cxa_atexit run-time regression with many registrations

When an array element is added or removed, make only the relevant page
writable, rather than the entire array. The entire array is still made
writable during recompaction and expansion.

This change fixes most of a large regression in __cxa_atexit runtime
(blueline, taskset 10, performance governor, 100000 registrations,
times are in seconds)
 - Q: _Exit=0.292380, exit=0.626801
 - R: _Exit=28.435082, exit=95.785110
 - new: _Exit=0.352285, exit=0.713893

Test: bionic unit tests
Test: adb shell taskset 10 \
  /data/benchmarktest64/bionic-spawn-benchmarks/bionic-spawn-benchmarks \
  --benchmark_filter='atexit' \
  --benchmark_display_aggregates_only=true \
  --benchmark_repetitions=10
Bug: http://b/168043760
Change-Id: I88cc15c29c9890b422b7f621f29f98a03ca1f886
Merged-In: I88cc15c29c9890b422b7f621f29f98a03ca1f886
(cherry picked from commit de523c02bbea1923ee5fbe9d1d60a905f6e965b0)