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
diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index 69638be..00322ec 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -266,7 +266,7 @@
   ASSERT_EQ(0, sigaction(SIGALRM, &action, &oldaction)) << strerror(errno);
-  run_watchpoint_test<Uint128_t>(watchpoint_imprecise_child, 8, 8);
+  run_watchpoint_test<Uint128_t>(watchpoint_imprecise_child, 8, sizeof(void*));
   ASSERT_EQ(0, sigaction(SIGALRM, &oldaction, nullptr)) << strerror(errno);