selftest/fpu: avoid clang warning

With extra warnings enabled, clang complains about the redundant
-mhard-float argument:

  clang: error: argument unused during compilation: '-mhard-float' [-Werror,-Wunused-command-line-argument]

Move this into the gcc-only part of the Makefile.

Link: https://lkml.kernel.org/r/20201203223652.1320700-1-arnd@kernel.org
Fixes: 4185b3b9

 ("selftests/fpu: Add an FPU selftest")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Cc: Borislav Petkov <bp@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>