Currently, all llvm-exegesis regression tests are target-specific and
they are organized so that the entire subdirectories of
llvm/test/tools/llvm-exegesis are enabled or disabled as a whole.
After the commit e0ad2af691 ("Skip codegen" dry-run mode), at least two
kinds of tests are possible:
- ensure that a snippet can be *generated* for the particular opcode
- tests involving actual snippet execution inside the llvm-exegesis process
Thus, for the particular target subdirectory, some tests should run only
on the particular host architecture and other tests only need the target
being among LLVM_TARGETS_TO_BUILD.
This commit defines a bunch of exegesis-specific features that can be
referenced in REQUIRES, UNSUPPORTED, etc. on a test-by-test basis.