llvmpipe: use global llvm context for PIPE_SUBSYSTEM_EMBEDDED

llvmpipe: use global llvm context for PIPE_SUBSYSTEM_EMBEDDED

There's 2 reasons why we'd want to use the global context:
1) There still seems to be one memory "leak" left when using multiple llvm
contexts (it is not a true leak as the memory disappears into some still
addressable pool but nevertheless the memory consumption grows). See
http://cgit.freedesktop.org/~jrfonseca/llvm-jitstress/
2) These contexts get kinda big - even when disposing modules etc. after
compiling a shader the LLVMContext can easily be over 100kB. So when there's
lots of llvm contexts arounds it adds up.

The downside is that at least right now this is absolutely not thread safe,
so this only works safely in environments where multiple pipe contexts are not
used concurrently.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>