x86: Compile builtin-copysign-8b.c with -mtune=generic

Compile builtin-copysign-8b.c with -mtune=generic so that it passes with

$ make check-gcc RUNTESTFLAGS="i386.exp=builtin-copysign-8b.c --target_board='unix{-m64\ -march=cascadelake}'"

	PR target/122323
	* gcc.target/i386/builtin-copysign-8b.c: Add -mtune=generic.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit 3ce0b19b87e9146c47343aab5088d144e739ce57)