Fix the FDE encoding to be relative on ELF.

This is a very late complement to r130637 which fixed this on x86_64. Fixes
pr15448.

Since it looks like that every elf architecture uses this encoding when using
cfi, make it the default for elf. Just exclude mips64el. It has a lovely
.ll -> .o test (ef_frame.ll) that tests that nothing changes in the binary
content of the .eh_frame produced by llc. Oblige it.