BpfRingbuf: Add support for 32 bit kernels

This change adds support for 32 bit kernels by capping mProducerPos to
4 bytes. mConsumerPos (which is written to by userspace) continues to
use 8 bytes. (This can be done because an entire page is allocated for
mConsumerPos to control access permissions). In a 32 bit kernel, that
means that the top order bits are just ignored. In addition, comparisons
in userspace only use the bottom 4 bytes to be bitness agnostic.

Test: atest BpfRingbufTest
Change-Id: I7fe6d9000a151512785f1aa2a53fa97d31967d19

staticlibs/native/bpf_headers/BpfRingbufTest.cpp

@@ -50,10 +50,6 @@ class BpfRingbufTest : public ::testing::Test {
       GTEST_SKIP() << "BPF ring buffers not supported below 5.8";
     }
 
-    if (!isKernel64Bit()) {
-      GTEST_SKIP() << "BPF ring buffers not supported on 32 bit kernel";
-    }
-
     errno = 0;
     mProgram.reset(retrieveProgram(mProgPath.c_str()));
     EXPECT_EQ(errno, 0);