[ELF] - Linkerscript: implemented SIZEOF(section)

SIZEOF(section)
Return the size in bytes of the named section, if that section has been allocated.

SIZEOF(section) often used in scripts. Few examples from the wild:
https://github.com/chipKIT32/pic32-Arduino-USB-Bootloader-original/blob/master/boot-linkerscript.ld
https://github.com/devkitPro/buildscripts/blob/master/dkarm-eabi/crtls/gba_cart.ld

Patch implements it.

Differential revision: https://reviews.llvm.org/D22915

llvm-svn: 277165