x86/fpu: Fully optimize out WARN_ON_FPU()

Currently WARN_ON_FPU evaluates its argument even if
CONFIG_X86_DEBUG_FPU is disabled, which adds unnecessary instructions to
several functions, for example kernel_fpu_begin().  Fix this by using
BUILD_BUG_ON_INVALID(x) in the no-debug case rather than (void)(x).

Fixes: 83242c5 ("x86/fpu: Make WARN_ON_FPU() more robust in the !CONFIG_X86_DEBUG_FPU case")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20250127224523.94300-1-ebiggers%40kernel.org

arch/x86/kernel/fpu/internal.h

@@ -18,7 +18,7 @@ static __always_inline __pure bool use_fxsr(void)
 #ifdef CONFIG_X86_DEBUG_FPU
 # define WARN_ON_FPU(x) WARN_ON_ONCE(x)
 #else
-# define WARN_ON_FPU(x) ({ (void)(x); 0; })
+# define WARN_ON_FPU(x) ({ BUILD_BUG_ON_INVALID(x); 0; })
 #endif
 
 /* Used in init.c */