Add 64-bit eh_frame section CFI support.

In practice, 64bit eh_frame is not used even for x86_64 binaries. The main reason is in eh_frame we almost always use pc-relative addressing, so addresses are within 32bits and gcc just sticks to 32bit eh_frame.

I generated 64bit eh_frame for Android Java runtime and unwind successfully in gdb, and in lldb with this patch.

Patch by Tong Shen.