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 778c048a8fc1a5485a53f670a587f09e4daf6bae)
diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index bce5898..19c63fb 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -260,7 +260,7 @@
   ASSERT_EQ(0, sigaction(SIGALRM, &action, &oldaction)) << strerror(errno);
   alarm(5);
 
-  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);
 }