gcc.target/i386/pr78904-4a.c: Compile with -mtune=generic

gcc.target/i386/pr78904-4a.c: Compile with -mtune=generic

commit e95395926a84a2406faefe0995295d199d595440
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Thu Jun 18 20:12:48 2020 +0200

    i386: Fix mode of ZERO_EXTRACT RTXes, remove ext_register_operand predicate.

caused

FAIL: gcc.target/i386/pr78904-4a.c scan-assembler [ \t]movb[\t ]+%.h, t

when compiled with --target_board='unix{-m32\ -march=cascadelake}'.  With
-mtune=generic:

	movzwl	4(%esp), %edx
	movl	8(%esp), %eax
	movb	%dh, t(%eax)
	ret

With -mtune=cascadelake:

	movzbl	5(%esp), %edx
	movl	8(%esp), %eax
	movb	%dl, t(%eax)
	ret

Add -mtune=generic for --target_board='unix{-m32\ -march=cascadelake}'.

	* gcc.target/i386/pr78904-4a.c: Compile with -mtune=generic.