[libcxx] Use relative path for libc++ library when generating script

[phosek]

Summary

This addresses the issue introduced in D60309 which leads to linker
scripts being generated with absolute paths.

[brzycki]

+1 to this patch. It fixes our nightly cross-compilers for arm and aarch64.

libcxx/utils/gen_link_script.py

def main():
     parser.add_argument("--rename", action="store_true", default=False,
                         help="Rename the output as input so we can replace it")
     parser.add_argument("--input", help="Path to libc++ library", required=True)
     parser.add_argument("--output", help="Path to libc++ linker script", required=True)
     parser.add_argument("libraries", nargs="+",
                         help="List of libraries libc++ depends on")
     args = parser.parse_args()
 
+    # Use the relative path for the libc++ library.
+    libcxx = os.path.relpath(args.input, os.path.dirname(args.output))
+
     # Prepare the list of public libraries to link.
     public_libs = ['-l%s' % l for l in args.libraries]
 
     # Generate the linker script contents.
-    contents = "INPUT(%s)" % ' '.join([args.input] + public_libs)
+    contents = "INPUT(%s)" % ' '.join([libcxx] + public_libs)
     print("GENERATING SCRIPT: '%s' as file %s" % (contents, args.output))
 
     if args.dryrun:
         return 0
 
     # Remove the existing libc++ symlink if it exists.
     if os.path.islink(args.output):
         os.unlink(args.output)