[ELF] - Linkerscript: support MIN and MAX.

Sample for the OVERLAY command from the spec 
(https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/sections.html)
uses MAX command that we do not support currently:

. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));

This patch implements support for MIN and MAX.

Differential revision: https://reviews.llvm.org/D44734

llvm-svn: 328696