[POWERPC] Fix for assembler -g

ppc64 does the unusual thing of using #include on a compiler-generated
assembly file (lparmap.s) from an assembly source file (head_64.S).
This runs afoul of my recent patch to pass -gdwarf2 to the assembler
under CONFIG_DEBUG_INFO.  This patch avoids the problem by disabling
DWARF generation (-g0) when producing lparmap.s.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>