Control Werror by converting to Wno-error (#30882)

Using `-Werror` is good practice for development and testing, but causes us a great
deal of heartburn supporting multiple compiler versions, especially as newer compiler
versions add warnings for released packages.  This PR adds support for suppressing
`-Werror` through spack's compiler wrappers.  There are currently three modes for
the `flags:keep_werror` setting:

* `none`: (default) cancel all `-Werror`, `-Werror=*` and `-Werror-*` flags by
  converting them to `-Wno-error[=]*` flags
* `specific`: preserve explicitly selected warnings as errors, such as
  `-Werror=format-truncation`, but reverse the blanket `-Werror`
* `all`: keeps all `-Werror` flags

These can be set globally in config.yaml, through the config command-line flags, or
overridden by a particular package (some packages use Werror as a proxy for determining
support for other compiler features).  We chose to use this approach because:

1. removing `-Werror` flags entirely broke *many* build systems, especially autoconf
   based ones, because of things like checking `-Werror=feature` and making the
   assumption that if that did not error other flags related to that feature would also work
2. Attempting to preserve `-Werror` in some phases but not others caused similar issues
3. The per-package setting came about because some packages, even with all these
   protections, still use `-Werror` unsafely.  Currently there are roughly 3 such packages
   known.

etc/spack/defaults/config.yaml

@@ -214,4 +214,9 @@ config:
 
   # Number of seconds a buildcache's index.json is cached locally before probing
   # for updates, within a single Spack invocation. Defaults to 10 minutes.
-  binary_index_ttl: 600
+  binary_index_ttl: 600
+
+  flags:
+    # Whether to keep -Werror flags active in package builds.
+    keep_werror: 'none'
+