[ELF] Fix handling of FDE negative relative PC addr

Signed values for the FDE PC addr were not correctly handled in
readFdeAddr(). If the value is negative and the type of the value is
smaller than 64 bits, the FDE PC addr overflow error would be
incorrectly triggered.

Fixed readFdeAddr() to properly handle signed values by sign extending
where appropriate.

Differential Revision: https://reviews.llvm.org/D49557

llvm-svn: 337683