ELF2: LinkerScript: lookup absolute paths under sysroot

In case a sysroot prefix is configured, and the filename starts with the
'/' character, and the script being processed was located inside the
sysroot prefix, the file's name will be looked for in the sysroot
prefix. Otherwise, the linker falls to the common lookup scheme.

https://www.sourceware.org/binutils/docs-2.24/ld/File-Commands.html

Differential Revision: http://reviews.llvm.org/D14916

llvm-svn: 254031