audio/carla: fix build on powerpc64le

Use the portable eel mode instead of dedicated assembly:
gmake[2]: Entering directory '/wrkdirs/usr/ports/audio/carla/work/Carla-2.5.0/source/modules/ysfx'
Compiling thirdparty/WDL/source/WDL/eel2/nseel-cfunc.c
{standard input}: Assembler messages:
{standard input}:1444: Error: syntax error; found (', expected ,'
{standard input}:1444: Error: junk at end of line: `(1048576)'
{standard input}:1445: Error: syntax error; found (', expected ,'
{standard input}:1445: Error: junk at end of line: `(1048576)'
{standard input}:1490: Error: syntax error; found (', expected ,'
{standard input}:1490: Error: junk at end of line: `(1048576)'
{standard input}:1491: Error: syntax error; found (', expected ,'
{standard input}:1491: Error: junk at end of line: `(1048576)'
{standard input}:2249: Error: missing operand