[llvm-exegesis] Improve test flexibility by using available_features

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 e0ad2af6 ("Skip codegen" dry-run mode), at least two
kinds of tests are possible:
1) ensure that a snippet can be *generated* for the particular opcode
2) 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.

~~

Huawei RRI

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D146302