compiler/clang: ignore main-return-type warning for clang

Unlike gcc, clang splits out the flag controlling warnings about the return
type of `main' from other warnings related to that function. Add the extra
-Wno-main-return-type flag to mask these warnings when building Zephyr
without -ffreestanding, as when using picolibc.

Signed-off-by: Keith Packard <keithp@keithp.com>