RISC-V: fix kernel CFLAGS with clang

Use the -march and -mabi flags for both gcc and clang as they are
compatible. Specify the "medium" code model separately as it goes by the
name "medany" under gcc, although they are equivalent.

Reviewed by: markj
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21436