model.lds.S: Quote file paths

Inserting arbitrary paths in a linker script verbatim can be
problematic, even if they don't contain whitespaces, as ld has a
special interpretation for certain special characters (such as @).

Fix this by quoting all user-provided paths in model.lds.S using the
preprocessor.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>