[PATCH] Make using non-default as/ld/etc easier.

[Fangrui Song]

On 2021-08-22, Olivier Galibert via Libc-alpha wrote:
>Add the configure parameters
>--with-{as,ld,ar,objdump,objcopy,gprof}=path to optionally force the
>path for these programs without having to tweak the output of
>$CC -print-prog-name in some way.
>
>In addition, when --with-{as,ld} is used the corresponding program
>version is not check.  The user gets to keep all the pieces if it
>blows.
>
>In practice, according to a build log on x86-64, $AS and $LD are never
>used directly.  $AR, $OBJDUMP and $OBJCOPY are though, and the llvm
>variants don't seem to have any issue.
>
>Signed-off-by: Olivier Galibert <galibert@pobox.com>

Thanks:) This will make
https://sourceware.org/pipermail/libc-alpha/2021-August/129864.html
("[PATCH v2 0/3] Allow LLD 13.0.0 and improve compatibility with gold and clang")
easier to use.

Currently I do
```
sudo ln -sf ~/Stable/bin/lld /usr/local/bin/ld
or
sudo ln -sf /usr/bin/ld.bfd /usr/local/bin/ld
``
when switching between GNU ld and LLD...