powerpc/xmon: Relax frame size for clang

When building with clang (8 trunk, 7.0 release) the frame size limit is
hit:

 arch/powerpc/xmon/xmon.c:452:12: warning: stack frame size of 2576
 bytes in function 'xmon_core' [-Wframe-larger-than=]

Some investigation by Naveen indicates this is due to clang saving the
addresses to printf format strings on the stack.

While this issue is investigated, bump up the frame size limit for xmon
when building with clang.

Link: https://github.com/ClangBuiltLinux/linux/issues/252


Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>