i386: wire up --with-tls to control -mtls-dialect= default

i386: wire up --with-tls to control -mtls-dialect= default

Allow passing --with-tls= at configure-time to control the default value
of -mtls-dialect= for i386 and x86_64. The default itself (gnu) is not changed
unless --with-tls= is passed.

--with-tls= is already wired up for ARM and RISC-V.

gcc/ChangeLog:
	PR target/120933

	* config.gcc (supported_defaults): Add tls for i386, x86_64.
	* config/i386/i386.h (host_detect_local_cpu): Add tls.
	* doc/install.texi: Document --with-tls= for i386, x86_64.