Makefile.uk: Silence warnings

Makefile.uk: Silence warnings

This commit changes several warning flags:
-Wno-error=sign-compare removed
-Wno-unused-value relegated to the glue code
-Wno-cast-function-type is sent to clang as well as GCC
-Wno-macro-redefined and -Wno-string-plus-int added for clang
-Wno-unused added for the entire library
-K&R-style declaration warnings silenced by -Wno-old-style-declaration
(GCC) or -Wno-deprecated-non-prototype (clang)
-Wno-unused-command-line-argument to silence assembler

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #61