bug#66984: target_alias variable set incorrectly

[Eugene Sharygin]

The bug manifests when trying to call `transform` on a tool name with a 
non-null
`target_triplet`.

Minimal code to reproduce:

> $ cat xyz.exp
> puts "size is [transform size]"
> $ runtest --target=powerpc-linux-gnu
> [snip]
> size is -size

> $ runtest --version
> DejaGnu version 1.6.3
> Expect version 5.45.4
> Tcl version 8.6

If run without a `--target` flag, it outputs "size is size", as 
expected.
However, if `--target` is specified, it prints "size is -size" instead 
of
the expected output, which is "powerpc-linux-gnu-size".

The reason being the `target_alias` variable, which is initially 
assigned [1] to
an empty string in `runtest.exp`, and which is then supposed to be 
defaulted
to the value of `target_triplet`, with the following code:

> if {![info exists target_alias]} {
>     set target_alias $target_triplet
> }

Of course, `info exists` returns 1 on a variable set to an empty string.

My proposed change is simply to omit the initial assignment of 
`target_alias`,
like this:

> --- runtest.exp.orig    2023-11-07 15:21:39.000000000 +0300
> +++ runtest.exp 2023-11-07 15:37:49.802466623 +0300
> @@ -82,7 +82,6 @@
>  set target_os     ""         ;# type of os the tests are running on
>  set target_vendor  ""		;# vendor name of the OS or workstation the 
> test are running on
>  set target_cpu     ""                ;# type of the cpu tests are running on
> -set target_alias   ""                ;# standard abbreviation of target
>  set compiler_flags ""                ;# the flags used by the compiler
>
>  #

[1]: 
http://git.savannah.gnu.org/gitweb/?p=dejagnu.git;a=blob;f=runtest.exp;h=84c7e28e117abff450e2aa9a8b9bbb1e618f0040;hb=HEAD#l85
[2]: 
http://git.savannah.gnu.org/gitweb/?p=dejagnu.git;a=blob;f=runtest.exp;h=84c7e28e117abff450e2aa9a8b9bbb1e618f0040;hb=HEAD#l890

-- Eugene