CPU Architecture support (#27924)

* CPU Architecture Support

This commit removes the `native` variant in favor of Spack's built-in support for specifying a target cpu architecture.  It also passes this information to the Legion build system so that it correctly passes the architecture to GASNet when built internally

* fixing whitespace

* Update package.py

based on a conversation with @streichler, this change sets `BUILD_MARCH` to an empty string, which will prevent legion's CMake build system from inserting `-march=native` and allow Spack to provide the correct architecture flags.

* Update package.py

adding a comment on what problem this MR solves.

* Update package.py

formatting

var/spack/repos/builtin/packages/legion/package.py

@@ -198,9 +198,6 @@ def validate_gasnet_root(value):
     variant('max_fields', values=int, default=512,
             description="Maximum number of fields allowed in a logical region.")
 
-    variant('native', default=False,
-            description="Enable native/host processor optimizaton target.")
-
     def cmake_args(self):
         spec = self.spec
         cmake_cxx_flags = []
@@ -339,10 +336,9 @@ def cmake_args(self):
             maxfields = maxfields << 1
         options.append('-DLegion_MAX_FIELDS=%d' % maxfields)
 
-        if '+native' in spec:
-            # default is off.
-            options.append('-DBUILD_MARCH:STRING=native')
-
+        # This disables Legion's CMake build system's logic for targeting the native
+        # CPU architecture in favor of Spack-provided compiler flags
+        options.append('-DBUILD_MARCH:STRING=')
         return options
 
     @run_after('install')