RISC-V: Add detailed comments on processing implied extensions. [NFC]

In some cases, we don't need to handle implied extensions. Add detailed
comments to help developers understand what implied ISAs should be
considered.

libgcc/ChangeLog:

	* config/riscv/feature_bits.c (__init_riscv_features_bits_linux):
	Add detailed comments on processing implied extensions.

Signed-off-by: Yangyu Chen <chenyangyu@isrc.iscas.ac.cn>