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