util/log: Add MESA_LOG_PREFIX environment variable to control log prefixes

Add a new MESA_LOG_PREFIX environment variable that allows fine-grained
control over log prefixes in file logger output on Linux. The variable
accepts a comma-separated list of options:
- "tag": include the tag prefix (e.g., "MESA:")
- "level": include the level prefix (e.g., "info:")

By default, both tag and level are included. Users can customize the
prefix by setting MESA_LOG_PREFIX to any combination (e.g., "tag",
"level", "tag,level", or empty string for no prefix), making the output
more flexible and readable for different use cases.

Other loggers (syslog, Android logcat, Windows debugger) are unaffected
and continue to include tag and level information as appropriate for
their format.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38217>

docs/envvars.rst

@@ -98,6 +98,10 @@ Core Mesa environment variables
    specifies a file name for logging all errors, warnings, etc., rather
    than stderr
 
+.. envvar:: MESA_LOG_PREFIX
+
+   specifies what to to include in the log prefix (linux only) - default is ``tag,level``
+
 .. envvar:: MESA_EXTENSION_OVERRIDE
 
    can be used to enable/disable extensions. A value such as