[ELF] Fix the file look up algorithm used in the linker script GROUP command.

In general the linker scripts's GROUP command works like a pair
of command line options --start-group/--end-group. But there is
a difference in the files look up algorithm.

The --start-group/--end-group commands use a trivial approach:
a) If the path has '-l' prefix, add 'lib' prefix and '.a'/'.so'
   suffix and search the path through library search directories.
b) Otherwise, use the path 'as-is'.

The GROUP command implements more compicated approach:
a) If the path has '-l' prefix, add 'lib' prefix and '.a'/'.so'
   suffix and search the path through library search directories.
b) If the path does not have '-l' prefix, and sysroot is configured,
   and the path starts with the / character, and the script being
   processed is located inside the sysroot, search the path under
   the sysroot. Otherwise, try to open the path in the current
   directory. If it is not found, search through library search
   directories.

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

The patch reviewed by Shankar Easwaran, Rui Ueyama.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207769 91177308-0d34-0410-b5e6-96231b3b80d8