amdfftw: fix broken build, adjust flags for performance tuning (#48754)

With CFLAGS, the code path in the amdfftw build system will bypass the logic around AMD_ARCH.

---------

Co-authored-by: vijay kallesh <Vijay-teekinavar.Kallesh@amd.com>

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

@@ -4,7 +4,6 @@
 
 import os
 
-from spack.build_environment import optimization_flags
 from spack.package import *
 from spack.pkg.builtin.fftw import FftwBase
 
@@ -95,14 +94,14 @@ class Amdfftw(FftwBase):
     )
     variant(
         "amd-dynamic-dispatcher",
-        default=True,
+        default=False,
         when="@4.1: %aocc@4.1.0:",
         description="Single portable optimized library"
         " to execute on different x86 CPU architectures",
     )
     variant(
         "amd-dynamic-dispatcher",
-        default=True,
+        default=False,
         when="@3.2: %gcc",
         description="Single portable optimized library"
         " to execute on different x86 CPU architectures",
@@ -203,11 +202,10 @@ def configure(self, spec, prefix):
         if not self.compiler.f77 or not self.compiler.fc:
             options.append("--disable-fortran")
 
-        # Cross compilation is supported in amd-fftw by making use of target
-        # variable to set AMD_ARCH configure option.
-        # Spack user can not directly use AMD_ARCH for this purpose but should
-        # use target variable to set appropriate -march option in AMD_ARCH.
-        options.append(f"AMD_ARCH={optimization_flags(self.compiler, spec.target)}")
+        if "avx512" in spec.target:
+            options.append("CFLAGS=-mprefer-vector-width=512")
+        else:
+            options.append("CFLAGS=-mprefer-vector-width=256")
 
         # Specific SIMD support.
         # float and double precisions are supported