cmake build system: change default build type to Release (#36679)

This switches the default Make build type to `build_type=Release`.

This offers:
- higher optimization level, including loop vectorization on older GCC
- adds NDEBUG define, which disables assertions, which could cause speedups if assertions are in loops etc
- no `-g` means smaller install size

Downsides are:
- worse backtraces (though this does NOT strip symbols)
- perf reports may be useless
- no function arguments / local variables in debugger (could be of course)
- no file path / line numbers in debugger

The downsides can be mitigated by overriding to `build_type=RelWithDebInfo` in `packages.yaml`,
if needed.  The upside is that builds will be MUCH smaller (and faster) with this change.

---------

Co-authored-by: Gregory Becker <becker33@llnl.gov>

lib/spack/spack/build_systems/cmake.py

@@ -90,9 +90,13 @@ class CMakePackage(spack.package_base.PackageBase):
 
     with when("build_system=cmake"):
         # https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html
+        # See https://github.com/spack/spack/pull/36679 and related issues for a
+        # discussion of the trade-offs between Release and RelWithDebInfo for default
+        # builds. Release is chosen to maximize performance and reduce disk-space burden,
+        # at the cost of more difficulty in debugging.
         variant(
             "build_type",
-            default="RelWithDebInfo",
+            default="Release",
             description="CMake build type",
             values=("Debug", "Release", "RelWithDebInfo", "MinSizeRel"),
         )