clover: Add environment variables for dumping kernel code v2

clover: Add environment variables for dumping kernel code v2

There are two debug variables:

CLOVER_DEBUG which you can set to any combination of llvm,clc,asm
(separated by commas) to dump llvm IR, OpenCL C, and native assembly.

CLOVER_DEBUG_FILE which you can set to a file name for dumping output
instead of stderr.  If you set this variable, the output will be split
into three separate files with different suffixes: .cl for OpenCL C,
.ll for LLVM IR, and .asm for native assembly.  Note that when data
is written, it is always appended to the files.

v2:
  - Code cleanups
  - Add CLOVER_DEBUG_FILE environment variable for dumping to a file.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>