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.

Change-Id: Ie96b4bc8ce0ad8c924ac737083cf9fe0182aee4d
Test: bionic-unit-tests-static --gtest_filter=sys_ptrace.*
Bug: 37662849