Don't leak ISA to __attribute__ ((target("arch=XXX")))

Don't leak ISA to __attribute__ ((target("arch=XXX")))

When processing __attribute__ ((target("arch=XXX"))), we should clear
the ISA bits in x_ix86_isa_flags first to avoid leaking ISA from
command line.

gcc/

	PR target/67995
	* config/i386/i386.c (ix86_valid_target_attribute_tree): If
	arch= is set,  clear all bits in x_ix86_isa_flags, except for
	ISA_64BIT, ABI_64, ABI_X32, and CODE16.

gcc/testsuite/

	PR target/67995
	* gcc.target/i386/pr67995-1.c: New test.
	* gcc.target/i386/pr67995-2.c: Likewise.
	* gcc.target/i386/pr67995-3.c: Likewise.

From-SVN: r228967