powerpc: Don't add -mabi= flags when building with Clang

When building pseries_defconfig, building vdso32 errors out:

  error: unknown target ABI 'elfv1'

This happens because -m32 in clang changes the target to 32-bit,
which does not allow the ABI to be changed.

Commit 4dc831aa ("powerpc: Fix compiling a BE kernel with a
powerpc64le toolchain") added these flags to fix building big endian
kernels with a little endian GCC.

Clang doesn't need -mabi because the target triple controls the
default value. -mlittle-endian and -mbig-endian manipulate the triple
into either powerpc64-* or powerpc64le-*, which properly sets the
default ABI.

Adding a debug print out in the PPC64TargetInfo constructor after line
383 above shows this:

  $ echo | ./clang -E --target=powerpc64-linux -mbig-endian -o /dev/null -
  Default ABI: elfv1

  $ echo | ./clang -E --target=powerpc64-linux -mlittle-endian -o /dev/null -
  Default ABI: elfv2

  $ echo | ./clang -E --target=powerpc64le-linux -mbig-endian -o /dev/null -
  Default ABI: elfv1

  $ echo | ./clang -E --target=powerpc64le-linux -mlittle-endian -o /dev/null -
  Default ABI: elfv2

Don't specify -mabi when building with clang to avoid the build error
with -m32 and not change any code generation.

-mcall-aixdesc is not an implemented flag in clang so it can be safely
excluded as well, see commit 238abecd ("powerpc: Don't use gcc
specific options on clang").

pseries_defconfig successfully builds after this patch and
powernv_defconfig and ppc44x_defconfig don't regress.

Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
[mpe: Trim clang links in change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191119045712.39633-2-natechancellor@gmail.com