GDB memory reads fail on Cortex-M33
Description of problem:
GDB fails to read memory from the guest.  There appear to be at least two problems:

1. In `arm_cpu_get_phys_page_attrs_debug`, `arm_is_secure(env)` returns false, because the implementation doesn't seem to know about Armv7-M or Armv8-M secure states.  However, `arm_mmu_idx(env)` does know how to check `env->v7m.secure`, so it returns `ARMMMUIdx_MSPriv` (the S stands for secure).  The mismatch between an apparently non-secure access to a secure MMU seems to cause the read to fail laster.
2. With the MPU enabled (not the case in this repro, but I can provide one), `cpu_memory_rw_debug` computes `page = addr & TARGET_PAGE_MASK`, and uses the page to compute permissions.  However, TARGET_PAGE_MASK is based on 4K pages on this platform, but the MPU granularity is 32 bytes.  So the wrong page is used for checking.
Steps to reproduce:
```
# Sorry for the large clone.  It's mostly unused files in CMSIS.
git clone --recursive -b qemu-repro-1 https://github.com/dreiss/mpu_experiments
cd mpu_experiments
git checkout origin/qemu-repro-1
cmake -S . -B build -DBOARD=qemu-mps2-an505 -DAPP=mpu_stacktrace -DCMAKE_BUILD_TYPE=Debug
cmake --build build
/path/to/qemu-system-arm -machine mps2-an505 -nographic -kernel build/kernel.elf -s -S -d int
# Open a separate terminal and cd into mpu_experiments
gdb build/kernel.elf -ex 'target remote :1234' -ex 'break base_case' -ex continue -ex backtrace -ex quit
# Note the memory read failures in the backtrace.
```