llvm-exegesis: Only run target specfic tests on native hosts

These tests were failing on SystemZ hosts, I don't think JIT'ing non-SystemZ
modules on SystemZ is supported.

The tests were initiallay enabled on all arches by
7a76140.

Differential Revision: https://reviews.llvm.org/D145763

(cherry picked from commit e30ce63)

Author: tstellar

llvm/test/tools/llvm-exegesis/PowerPC/lit.local.cfg

@@ -1,3 +1,6 @@
 if not ('PowerPC' in config.root.targets):
     # We need support for PowerPC.
     config.unsupported = True
+elif not ('powerpc' in config.root.host_triple):
+    # We need to be running on an PPC host.
+    config.unsupported = True

llvm/test/tools/llvm-exegesis/X86/lit.local.cfg

@@ -1,3 +1,6 @@
 if not ('X86' in config.root.targets):
     # We need support for X86.
     config.unsupported = True
+elif not ('x86_64' in config.root.host_triple):
+    # We need to be running on an X86 host.
+    config.unsupported = True