* gnu/packages/gl.scm (mesa)[inputs]: Add llvm-11 on riscv64-linux.
[native-inputs]: Add glslang on riscv64-linux.
[arguments]: In configure-flags adjust gallium-drivers to swap the
power* and intel drivers for the fallback option. Specify riscv64-linux
configure options for vulkan-drivers. Enable the vulkan-overlay-layer
and llvm support. Adjust the custom 'disable-failing-test phase to have
a riscv64-linux section.