gallium/os: call "ANSI" version of GetCommandLine

gallium/os: call "ANSI" version of GetCommandLine

The GetCommandLine API comes in two versions, GetCommandLineA (which
returns "ANSI" results), and GetCommandLineW which returns UTF-16
("WIDE") results. Then finally, windows.h provides a wrapper-macro that
defines GetCommandLine to either of the two, based on the setting of
the UNICODE macro.

More information about this mechanism can be found here:
https://docs.microsoft.com/en-us/windows/win32/intl/unicode-in-the-windows-api

For some reason, the UNICODE macro is set during build, even if we're
not explicitly setting it. This leads to us trying to cast a UTF-16
result to a char-pointer, which is obviously not going to do the right
thing.

So let's be defensive, and just call GetCommandLineA directly instead.
This avoids us depending on the setting of the UNICODE-macro in the
first place.

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5497>