driver: Fix multilib_os_dir and multiarch_dir for those target use TARGET_COMPUTE_MUL...

driver: Fix multilib_os_dir and multiarch_dir for those target use TARGET_COMPUTE_MULTILIB

This patch fixes the multilib_os_dir and multiarch_dir for those targets
that use TARGET_COMPUTE_MULTILIB, since the TARGET_COMPUTE_MULTILIB hook
only update/fix the multilib_dir but not the multilib_os_dir and multiarch_dir,
so the multilib_os_dir and multiarch_dir are not set correctly for those targets.

Use RISC-V linux target (riscv64-unknown-linux-gnu) as an example:

```
$ riscv64-unknown-linux-gnu-gcc -print-multi-lib
.;
lib32/ilp32;@march=rv32imac@mabi=ilp32
lib32/ilp32d;@march=rv32imafdc@mabi=ilp32d
lib64/lp64;@march=rv64imac@mabi=lp64
lib64/lp64d;@march=rv64imafdc@mabi=lp64d
```

If we use the exactly same -march and -mabi options to compile a source file,
the multilib_os_dir and multiarch_dir are set correctly:

```
$ riscv64-unknown-linux-gnu-gcc -print-multi-os-directory -march=rv64imafdc -mabi=lp64d
../lib64/lp64d
$ riscv64-unknown-linux-gnu-gcc -print-multi-directory -march=rv64imafdc -mabi=lp64d
lib64/lp64d
```

However if we use the -march=rv64imafdcv -mabi=lp64d option to compile a source
file, the multilib_os_dir and multiarch_dir are not set correctly:
```
$ riscv64-unknown-linux-gnu-gcc -print-multi-os-directory -march=rv64imafdc -mabi=lp64d
lib64/lp64d
$ riscv64-unknown-linux-gnu-gcc -print-multi-directory -march=rv64imafdc -mabi=lp64d
lib64/lp64d
```

That's because the TARGET_COMPUTE_MULTILIB hook only update/fix the multilib_dir
but not the multilib_os_dir, so the multilib_os_dir is blank and will use same
value as multilib_dir, but that is not correct.

So we introduce second chance to fix the multilib_os_dir if it's not set, we do
also try to fix the multiarch_dir, because it may also not set correctly if
multilib_os_dir is not set.

Changes since v1:
- Fix non-multilib build.
- Fix fix indentation.

gcc/ChangeLog:

	* gcc.cc (find_multilib_os_dir_by_multilib_dir): New.
	(set_multilib_dir): Fix multilib_os_dir and multiarch_dir
	if multilib_os_dir is not set.