gallium/util: Don't use __builtin_clrsb in util_last_bit().
gallium/util: Don't use __builtin_clrsb in util_last_bit().

Unclear circumstances lead to undefined symbols on x86.

Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=536916
Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org
Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler