doc: RISC-V: Document that -mcpu doesn't override -march or -mtune

This came up recently as Edwin was looking through the test suite.  A
few of us were talking about this during the patchwork meeting and were
surprised.  Looks like this is the desired behavior, so let's at least
document it.

gcc/ChangeLog:

	* doc/invoke.texi: Document -mcpu.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>