gallivm: (trivial) fix copy/paste bug with big endian code
gallivm: (trivial) fix copy/paste bug with big endian code

8bd67a35c50e68c21aed043de11e095c284d151a introduced using undefined variable
on big endian archs due to copy/paste bug.
(compile hack tested only)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler