Allow targets to override generic implementations

Targets can override generic implementations by adding a file called
OVERRIDES in $(TARGET_DIR)/lib and listing the generic implementations
that it wants to override.  For example, to override get_group_id() and
get_global_size() you would add these lines to the OVERRIDES file:

workitem/get_group_id.cl
workitem/get_global_size.cl

llvm-svn: 184982