AddressIsReadable: improve comments

Linux kernel's rt_sigprocmask correctly handles an unaligned user address[1].
The original issue was for qemu-user, which seems long irrelevant.

Tested locally on an AArch64 CPU and qemu-aarch64-static.

The alignment operation actually serves another purpose: when addr resides in
the last 7 bytes of a page (unaligned), check only the current page and not the
next. Update the comment.

[1]:
kernel/signal.c `SYSCALL_DEFINE4(rt_sigprocmask`
arch/arm64/include/asm/uaccess.h:raw_copy_from_user
arch/arm64/lib/copy_template.S  "alignment handled by the hardware"

PiperOrigin-RevId: 592618320
Change-Id: Ifbd05aba42f46e36e710cca940570213036b3ce0

absl/debugging/internal/address_is_readable.cc

@@ -50,8 +50,10 @@ namespace debugging_internal {
 // NOTE: any new system calls here may also require sandbox reconfiguration.
 // NOTE: any new system calls here may also require sandbox reconfiguration.
 //
 //
 bool AddressIsReadable(const void *addr) {
 bool AddressIsReadable(const void *addr) {
-  // Align address on 8-byte boundary. On aarch64, checking last
-  // byte before inaccessible page returned unexpected EFAULT.
+  // rt_sigprocmask below checks 8 contiguous bytes. If addr resides in the
+  // last 7 bytes of a page (unaligned), rt_sigprocmask would additionally
+  // check the readability of the next page, which is not desired. Align
+  // address on 8-byte boundary to check only the current page.
   const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7};
   const uintptr_t u_addr = reinterpret_cast<uintptr_t>(addr) & ~uintptr_t{7};
   addr = reinterpret_cast<const void *>(u_addr);
   addr = reinterpret_cast<const void *>(u_addr);