m68k/setjmp.S: fix build with gcc 3.4.6 -std=gnu99

The register prefix is always % for the architecture we use (elf),
hardcode it instead of confusing the preprocessor

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@28025 a1c6a512-1295-4272-9138-f99709370657
1 file changed