powerpc: Work around inline asm issues in alternate feature sections

The clang toolchain treats inline assembly a bit differently than
straight assembly code. In particular, inline assembly doesn't have
the complete context available to resolve expressions. This is
intentional to avoid divergence in the resulting assembly code.

We can work around this issue by borrowing a workaround done for ARM,
i.e. not directly testing the labels themselves, but by moving the
current output pointer by a value that should always be zero. If this
value is not null, then we will trigger a backward move, which is
explicitly forbidden.

Signed-off-by: Bill Wendling <morbo@google.com>
[mpe: Put it in a macro and only do the workaround for clang]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201120224034.191382-4-morbo@google.com