PowerPC64 stub: on syscall, real Linux trashes r13 and below

(except r1, r2, and input args!) So, r12 .e_entry for musl
must copy from a saved register.  [qemu-ppc64 doesn't trash registers!]
	modified:   ../.github/travis_testsuite_1.sh
	modified:   p_lx_elf.cpp
	modified:   stub/src/amd64-linux.elf-main.c
	modified:   stub/src/powerpc64le-linux.elf-fold.S
	also .h  .fold.map

src/p_lx_elf.cpp

@@ -473,6 +473,7 @@ off_t PackLinuxElf64::pack3(OutputFile *fo, Filter &ft)
         upx_uint64_t const pm = (
                Elf64_Ehdr::EM_X86_64 ==e_machine
             || Elf64_Ehdr::EM_AARCH64==e_machine
+            //|| Elf64_Ehdr::EM_PPC64  ==e_machine  /* DOES NOT WORK! */
             )
             ? ((~(upx_uint64_t)0)<<12)
             : page_mask;