[Bug target/84797] RISC-V: add --with-multilib-list support

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84797

--- Comment #2 from Jim Wilson <wilson at gcc dot gnu.org> ---
Created attachment 43904
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43904&action=edit
version 2 patch

Missing documentation, doesn't handle architecture aliases, and only lets your
specify one ABI, but otherwise seems to be working right.  Configuring with
    --enable-multilib --with-multilib-list=lp64d --with-abi=lp64d
--with-arch=rv64gc
I get a compiler that does this

gamma05:2013$ ./xgcc -B./ --print-multi-lib
.;
gamma05:2014$ ./xgcc -B./ --print-multi-dir
.
gamma05:2015$ ./xgcc -B./ --print-multi-os-directory
../lib64/lp64d
gamma05:2016$ 

So one multilib was built, and it was installed into lib64/lp64d where we want
it.  The --print-multi-dir value shouldn't matter.

Most ports only select multilibs based on two options.  The RISC-V port uses
two options to select multilibs, which makes specifying this stuff a lot more
complicated.  This is why I'm only allowing one ABI choice at the moment.

I see I forgot to cleanup the t-linux-withmultilib file.  I will do that in the
next version of the patch.