riscv: gitlab-ci: Add clang build tests

Test building 32 and 64-bit with clang. Throw a test of in- and out-
of-tree building in too by swapping which is done to which (32-bit
vs. 64-bit) with respect to the gcc build tests.

Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>