[ELF] - Stop silently producing a broken .eh_frame_hdr.

Currently, getFdePC() returns uint64_t. Its because the following
encodings might use 8 bytes: DW_EH_PE_absptr and DW_EH_PE_udata8.

But caller assigns returned value to uint32_t field:
https://github.com/llvm-mirror/lld/blob/master/ELF/SyntheticSections.cpp#L508

Value is used for building .eh_frame_hdr section.
We use DW_EH_PE_sdata4 encoding for building it at this moment:
https://github.com/llvm-mirror/lld/blob/master/ELF/SyntheticSections.cpp#L2545

And that means that an overflow issue might happen if
DW_EH_PE_absptr/DW_EH_PE_udata8 address encodings are present
in .eh_frame. In that case, before this patch, we silently would
truncate the address and produced broken .eh_frame_hdr section.

It would be not hard to support real 64-bit values for
DW_EH_PE_absptr/DW_EH_PE_udata8 encodings, but it is
unclear if it is usefull and if we should do it.

Since nobody faced/reported it, int this patch I only implement
a check to stop producing broken output silently for now.

llvm-svn: 337382