crt: Fix building lib32+lib64 in parallel

In 0ad299826ca14987fd53664c1047f4dfe848c3f8, we started passing
--temp-prefix to dlltool (if supported), to avoid temp file clashes
when running on windows.

We used the file name portion of the output file path as temp prefix;
e.g. when building lib64/libuser32.a, we ended up passing
"--temp-prefix libuser32" to dlltool. This avoids the temp file
clashes on Windows, since there's no randomness involved in the file
names.

However, if both lib32 and lib64 are enabled at the same time,
we can end up building e.g. lib32/libuser32.a and lib64/libuser32.a
in parallel, which both would end up with the same temp file prefix,
which then causes build failures.

Therefore, pass the whole output name, minus extension, as temp prefix,
i.e. passing "--temp-prefix lib64/libuser32" as prefix. In practice,
this produces the temp object files in the subdirectory, while keeping
their names the same.

For the practical details of the change; previously this expanded into
a subshell, calling the shell utility "basename", which removes both
leading directories and an optional suffix. Switch this to use the
gnu make builtin function "basename" instead, which keeps the directory
prefix, but removes any suffixes.

Signed-off-by: Martin Storsjö <martin@martin.st>