watchpoint_imprecise test: set 4-byte watchpoints on 32-bit arches

8-byte watchpoints are not supported on 32-bit kernels (at least on x86,
probably arm as well). 8-byte watchpoints are not required for this
test, so just decrease the size of watchpoints on these architectures.

Test: bionic-unit-tests-static --gtest_filter=sys_ptrace.*
Bug: 37662849
Change-Id: I450a78fc1d6b8807fe1c776f5fa62d329788969e
(cherry picked from commit 778c048a)