[ELF] - implement support of extended length field for CIE/FDE records of eh_frame.

Ian Lance Taylor writes: "Read 4 bytes. If they are not 0xffffffff, they are the length of the CIE or FDE record. Otherwise the next 64 bits holds the length, and this is a 64-bit DWARF format. This is like .debug_frame." (http://www.airs.com/blog/archives/460), that also consistent with spec (https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-PDA/LSB-PDA/ehframechpt.html).

Patch implements support of described extended length field and also adds few more checks for safety.

Differential revision: http://reviews.llvm.org/D15532

llvm-svn: 255883