backends: Handle SHT_X86_64_UNWIND as valid relocation target type.

The x86_64 abi defines a special section type for .eh_frame[_hdr],
SHT_X86_64_UNWIND, which is a valid relocation target type.

https://sourceware.org/bugzilla/show_bug.cgi?id=26878

Signed-off-by: Mark Wielaard <mark@klomp.org>
3 files changed