[bazel] Respect llvm_target_list in llvm-exegesis

[rupprecht]

Summary

• 47afaf2eb02b1424a9aba241ccd02393a0cbc648 changed llvm-exegesis cmake rules
• 5b2f838db42ea190fdda147a33b5f0fcc8145689 ported them to bazel, but did so by adding all the lib/{target}/*.cpp sources in exegesis to the build rule
• c7bf9d084d037aa6c8fd479be9ccdf963dc59e10 removed it, because it breaks users who don't build Mips and fail when building lib/Mips/*.cpp. But that in turn breaks those who *do* build the Mips target.

This should hopefully fix it for the final time by using selectively build subdirectories of exegesis target libs using llvm_target_exegesis, which is derived from llvm_targets, and is the list that can vary based on the downstream user.

I verified this builds with and without Mips in the DEFAULT_TARGETS configure list, and also double checked with bazel query --output=build @llvm-project//llvm:Exegesis that lib/Mips/Target.cpp is being included if and only if Mips is in the target list.

[aeubanks]

(I think submitting this without review would have been fine)

[rupprecht]

In D134512#3810664, @aeubanks wrote:

(I think submitting this without review would have been fine)

That's what I thought about 5b2f838db42ea190fdda147a33b5f0fcc8145689, so I figured the second attempt should have a pair of eyes :)

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

deps = [
         ":Support",
     ],
 )
 
 cc_library(
     name = "Exegesis",
     srcs = glob([
         "tools/llvm-exegesis/lib/*.cpp",
-        "tools/llvm-exegesis/lib/AArch64/*.cpp",
-        "tools/llvm-exegesis/lib/PowerPC/*.cpp",
-        "tools/llvm-exegesis/lib/X86/*.cpp",
-        "tools/llvm-exegesis/lib/X86/*.h",
         # We have to include these headers here as well as in the `hdrs` below
         # to allow the `.cpp` files to use file-relative-inclusion to find
         # them, even though consumers of this library use inclusion relative to
         # `tools/llvm-exegesis/lib` with the `strip_includes_prefix` of this
         # library. This mixture appears to be incompatible with header modules.
         "tools/llvm-exegesis/lib/*.h",
+    ] + [
+        "tools/llvm-exegesis/lib/{}/*.cpp".format(t)
+        for t in llvm_target_exegesis
+    ] + [
+        "tools/llvm-exegesis/lib/{}/*.h".format(t)
+        for t in llvm_target_exegesis
     ]),
     hdrs = glob(["tools/llvm-exegesis/lib/*.h"]),
     copts = llvm_copts + ["-DHAVE_LIBPFM=1"],
     features = ["-header_modules"],
     strip_include_prefix = "tools/llvm-exegesis/lib",
     tags = [
         "manual",  # External dependency (libpfm4)
         "nobuildkite",  # TODO(chandlerc): Add support for fetching and building libpfm4 and enable this.