x86,bugs: Unconditionally allow spectre_v2=retpoline,amd

Currently Linux prevents usage of retpoline,amd on !AMD hardware, this
is unfriendly and gets in the way of testing. Remove this restriction.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.487348118@infradead.org

arch/x86/kernel/cpu/bugs.c

@@ -882,13 +882,6 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
  return SPECTRE_V2_CMD_AUTO;
  }
 
- if (cmd == SPECTRE_V2_CMD_RETPOLINE_AMD &&
- boot_cpu_data.x86_vendor != X86_VENDOR_HYGON &&
- boot_cpu_data.x86_vendor != X86_VENDOR_AMD) {
- pr_err("retpoline,amd selected but CPU is not AMD. Switching to AUTO select\n");
- return SPECTRE_V2_CMD_AUTO;
- }
-
  spec_v2_print_cond(mitigation_options[i].option,
  mitigation_options[i].secure);
  return cmd;