Make macOS installed libraries more relocatable (#26608)

* relocate: call install_name_tool less

* zstd: fix race condition

Multiple times on my mac, trying to install in parallel led to failures
from multiple tasks trying to simultaneously create `$PREFIX/lib`.

* PackageMeta: simplify callback flush

* Relocate: use spack.platforms instead of platform

* Relocate: code improvements

* fix zstd

* Automatically fix rpaths for packages on macOS

* Only change library IDs when the path is already in the rpath

This restores the hardcoded library path for GCC.

* Delete nonexistent rpaths and add more testing

* Relocate: Allow @executable_path and @loader_path

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

@@ -37,10 +37,10 @@ class Zstd(MakefilePackage):
     depends_on('lzma', when='+programs')
     depends_on('lz4', when='+programs')
 
-    def _make(self, *args):
+    def _make(self, *args, **kwargs):
         # PREFIX must be defined on macOS even when building the library, since
         # it gets hardcoded into the library's install_path
-        make('VERBOSE=1', 'PREFIX=' + self.prefix, '-C', *args)
+        make('VERBOSE=1', 'PREFIX=' + self.prefix, '-C', *args, **kwargs)
 
     def build(self, spec, prefix):
         self._make('lib')
@@ -48,6 +48,6 @@ def build(self, spec, prefix):
             self._make('programs')
 
     def install(self, spec, prefix):
-        self._make('lib', 'install')
+        self._make('lib', 'install', parallel=False)
         if spec.variants['programs'].value:
             self._make('programs', 'install')