x86/build: Clean up stack alignment flags in CC_FLAGS_FPU

x86/build: Clean up stack alignment flags in CC_FLAGS_FPU

The minimum supported version of GCC to build the x86 kernel was bumped
to GCC 8.1 in

  a3e8fe814ad1 ("x86/build: Raise the minimum GCC version to 8.1").

Prior to GCC 7.1, '-mpreferred-stack-boundary=3' was not allowed with
'-msse', so areas of the kernel that needed floating point had a
different alignment. Now that GCC > 7.1 is mandatory, there is no need
to have a different value of '-mpreferred-stack-boundary' from the rest
of the kernel, so remove this handling from CC_FLAGS_FPU.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=34fac449e121be97dd073c5428cc855367b2872c
Link: https://lore.kernel.org/20250814-x86-min-ver-cleanups-v1-3-ff7f19457523@kernel.org