[ELF] - Linkerscript: implemented BYTE/SHORT/LONG/QUAD commands.

The BYTE, SHORT, LONG, and QUAD commands store one, two, four, and eight bytes (respectively). 
After storing the bytes, the location counter is incremented by the number of bytes
stored.

Previously our scripts handles these commands incorrectly. For example:
SECTIONS  {
  .foo : {
 *(.foo.1)
 BYTE(0x11)
...
We accepted the script above treating BYTE as input section description. 
These commands are used in the wild though.

Differential revision: https://reviews.llvm.org/D24830

llvm-svn: 282429