Kludge to support linux-user on a PPC64

Some headers (e.g. signal.h) include asm/elf.h and the chaos of macro
redefinitions ensues, this kludge avoids it.

Alternative way to fight that would be to change all ELF_XXX and some
other definitions to QEMU_ELF_XXX or something to that effect.

This patch concludes the quest for enabling linux-user on PPC64,
however, since qemu_mallocz uses mmap and, at least on this system,
mmap tends to return values that do not fit into 32bit, emulation for
32bit targets does not work without some hacks wich are to disgusting
to commit (and as `man mmap' tells us MAP_32BIT is only implemented on
X86_64)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5037 c046a42c-6fe2-441c-8c8c-71466251a162
