powerpc: Fix stack protector Kconfig test for clang

powerpc: Fix stack protector Kconfig test for clang

Clang's in-progress per-task stack protector support [1] does not work
with the current Kconfig checks because '-mstack-protector-guard-offset'
is not provided, unlike all other architecture Kconfig checks.

  $ fd Kconfig -x rg -l mstack-protector-guard-offset
  ./arch/arm/Kconfig
  ./arch/riscv/Kconfig
  ./arch/arm64/Kconfig

This produces an error from clang, which is interpreted as the flags not
being supported at all when they really are.

  $ clang --target=powerpc64-linux-gnu \
          -mstack-protector-guard=tls \
          -mstack-protector-guard-reg=r13 \
          -c -o /dev/null -x c /dev/null
  clang: error: '-mstack-protector-guard=tls' is used without '-mstack-protector-guard-offset', and there is no default

This argument will always be provided by the build system, so mirror
other architectures and use '-mstack-protector-guard-offset=0' for
testing support, which fixes the issue for clang and does not regress
support with GCC.

Even with the first problem addressed, the 32-bit test continues to fail
because Kbuild uses the powerpc64le-linux-gnu target for clang and
nothing flips the target to 32-bit, resulting in an error about an
invalid register valid:

  $ clang --target=powerpc64le-linux-gnu \
          -mstack-protector-guard=tls
          -mstack-protector-guard-reg=r2 \
          -mstack-protector-guard-offset=0 \
          -x c -c -o /dev/null /dev/null
  clang: error: invalid value 'r2' in 'mstack-protector-guard-reg=', expected one of: r13

While GCC allows arbitrary registers, the implementation of
'-mstack-protector-guard=tls' in LLVM shares the same code path as the
user space thread local storage implementation, which uses a fixed
register (2 for 32-bit and 13 for 62-bit), so the command line parsing
enforces this limitation.

Use the Kconfig macro '$(m32-flag)', which expands to '-m32' when
supported, in the stack protector support cc-option call to properly
switch the target to a 32-bit one, which matches what happens in Kbuild.
While the 64-bit macro does not strictly need it, add the equivalent
64-bit option for symmetry.

Cc: stable@vger.kernel.org # 6.1+
Link: https://github.com/llvm/llvm-project/pull/110928
Reviewed-by: Keith Packard <keithp@keithp.com>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241009-powerpc-fix-stackprotector-test-clang-v2-1-12fb86b31857@kernel.org