[ELF] - R_X86_64_SIZE64/R_X86_64_SIZE32 relocations implemented.

R_X86_64_SIZE64/R_X86_64_SIZE32 relocations were introduced in 0.98v of "System V Application Binary Interface x86-64" (http://www.x86-64.org/documentation/abi.pdf).

Calculation for them is Z + A, where:
Z - Represents the size of the symbol whose index resides in the relocation entry.
A - Represents the addend used to compute the value of the relocatable field.

Differential revision: http://reviews.llvm.org/D15335

llvm-svn: 255332