Revert code change of D63497 & D74399 for riscv64-*-linux GCC detection

This partially reverts commits 1fc2a47f and 9816e726

.

See D109727. Replacing config.guess in favor of {gcc,clang} -dumpmachine
can avoid the riscv64-{redhat,suse}-linux GCC detection.

Acked-by: Luís Marques <luismarques@lowrisc.org>