x86/bugs: Use early_param() for spectre_v2

Most of the mitigations in bugs.c use early_param() for command line parsing.
Rework the spectre_v2 and nospectre_v2 command line options to be
consistent with the others.

Remove spec_v2_print_cond() as informing the user of the their cmdline
choice isn't interesting.

  [ bp: Zap spectre_v2_check_cmd(). ]

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/r/20250819192200.2003074-3-david.kaplan@amd.com