build: meson: Remove unnecessary auxiliary variable

The variable containing the list of compiler arguments to be checked
can be removed without any harm to readibility.

The variable has been removed by appending directly to the list of
common compiler arguments, those that are supported.

meson.build

@@ -148,7 +148,7 @@ if enable_ld_gc
 endif
 
 if nm_debug
-  test_cflags = [
+  common_flags += cc.get_supported_arguments([
     '-fno-strict-aliasing',
     '-Wdeclaration-after-statement',
     '-Wfloat-equal',
@@ -172,9 +172,7 @@ if nm_debug
     '-Wtypedef-redefinition',
     '-Wundef',
     '-Wunknown-attributes',
-  ]
-
-  common_flags += cc.get_supported_arguments(test_cflags)
+  ])
 endif
 
 add_project_arguments(common_flags, language: 'c')