build: Declare __LIBNAME__, __BASENAME__ and __VARIANT__ for .lds.S

build: Declare __LIBNAME__, __BASENAME__ and __VARIANT__ for .lds.S

.lds.S files are linker script files that use the C preprocessor in order
to generate the final .lds file. This commit provides Unikraft's common
declarations of `__LIBNAME__`, `__BASENAME__`, `__VARIANT__` to this
pre-processing step so that they can be used for generating linker scripts.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Delia Pavel <delia_maria.pavel@stud.acs.upb.ro>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #868