[BOLT][test] Enable cross-target testing

[maksfb]

Summary

Check for supported target architecture instead of the host arch when
deciding to execute non-runtime tests.

[maksfb]

Note that some tests needs adjustment (or moved under /runtime) before this diff could be landed.

[yota9]

Great patch, thank you!

[maksfb]

Rebase

bolt/test/AArch64/lit.local.cfg

-if config.host_arch not in ['aarch64']:
+if 'AArch64' not in config.root.targets:
     config.unsupported = True
 
 config.substitutions.insert(
     0, ('%cflags',
         '%cflags --target=aarch64-pc-linux -nostartfiles -nostdlib -fuse-ld=lld'
         ' -Wl,--unresolved-symbols=ignore-all'))

bolt/test/X86/lit.local.cfg

-if config.host_arch not in ['x86', 'X86', 'x86_64']:
+if not 'X86' in config.root.targets:
     config.unsupported = True
 
 config.substitutions.insert(
     0, ('%cflags',
         '%cflags --target=x86_64--pc-linux -nostartfiles -nostdlib -fuse-ld=lld'
         ' -Wl,--unresolved-symbols=ignore-all'))

bolt/test/lit.cfg.py

def calculate_arch_features(arch_string):
     return features
 
 
 llvm_config.feature_config(
     [('--assertion-mode', {'ON': 'asserts'}),
      ('--cxxflags', {r'-D_GLIBCXX_DEBUG\b': 'libstdcxx-safe-mode'}),
         ('--targets-built', calculate_arch_features)
      ])
+
+config.targets = frozenset(config.targets_to_build.split())

bolt/test/lit.site.cfg.py.in

 config.llvm_use_sanitizer = "@LLVM_USE_SANITIZER@"
 config.enable_shared = @ENABLE_SHARED@
 config.enable_backtrace = @ENABLE_BACKTRACES@
 config.host_arch = "@HOST_ARCH@"
 config.enable_abi_breaking_checks = "@LLVM_ENABLE_ABI_BREAKING_CHECKS@"
 config.python_executable = "@PYTHON_EXECUTABLE@"
 config.bolt_clang = "@BOLT_CLANG_EXE@"
 config.bolt_lld = "@BOLT_LLD_EXE@"
+config.targets_to_build = "@TARGETS_TO_BUILD@"
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@BOLT_SOURCE_DIR@/test/lit.cfg.py")