RISC-V: Don't unconditionally add m,a,f,d in arch-canonicalize

RISC-V: Don't unconditionally add m,a,f,d in arch-canonicalize

This solves an issue where rv32i, etc. are canonicalized to rv32imafd
since the g->i addition of 'm', 'a', 'f', 'd' is not actually gated by
whether the input was rv32g/rv64g.

gcc/ChangeLog:

	* config/riscv/arch-canonicalize: Only add mafd extension if
	base was rv32/rv64g.