Hey Guix,

this patch series adds Vulkan support for mesa on x86_64-linux and adds 
a few new packages.
Since a rebuild of mesa is necessary I'm sending it in for core-updates.

Here's a short explanation of all the patches:
#1: Add Vulkan support to mesa on x86_64-linux. On i686-linux mesa 
didn't build unfortunately.
I haven't tried other architectures.
#2: Add vulkan.scm to gnu/local.mk.
#3: Create vulkan.scm and add spirv-headers.
#4: Add spirv-tools to vulkan.scm.
#5: Add glslang to vulkan.scm.
#6: Add vulkan-icd-loader to vulkan.scm.
This provides the 'vulkaninfo' tool that you can use to query if your 
system is Vulkan-capable.

I tested it out after a full rebuild on a Vulkan-capable system.
'vulkaninfo' showed Vulkan was available on that system.