x86/build: Disable HIGHMEM64G selection for M486SX

Fix a regression caused by making the 486SX separately selectable in
Kconfig, for which the HIGHMEM64G setting has not been updated and
therefore has become exposed as a user-selectable option for the M486SX
configuration setting unlike with original M486 and all the other
settings that choose non-PAE-enabled processors:

  High Memory Support
  > 1. off (NOHIGHMEM)
    2. 4GB (HIGHMEM4G)
    3. 64GB (HIGHMEM64G)
  choice[1-3?]:

With the fix in place the setting is now correctly removed:

  High Memory Support
  > 1. off (NOHIGHMEM)
    2. 4GB (HIGHMEM4G)
  choice[1-2?]:

 [ bp: Massage commit message. ]

Fixes: 87d6021b ("x86/math-emu: Limit MATH_EMULATION to 486SX compatibles")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org # v5.5+
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.2104141221340.44318@angie.orcam.me.uk

arch/x86/Kconfig

@@ -1406,7 +1406,7 @@ config HIGHMEM4G
 
 config HIGHMEM64G
 	bool "64GB"
-	depends on !M486 && !M586 && !M586TSC && !M586MMX && !MGEODE_LX && !MGEODEGX1 && !MCYRIXIII && !MELAN && !MWINCHIPC6 && !WINCHIP3D && !MK6
+	depends on !M486SX && !M486 && !M586 && !M586TSC && !M586MMX && !MGEODE_LX && !MGEODEGX1 && !MCYRIXIII && !MELAN && !MWINCHIPC6 && !WINCHIP3D && !MK6
 	select X86_PAE
 	help
 	  Select this if you have a 32-bit processor and more than 4