MIPS: Move '-Wa,-msoft-float' check from as-option to cc-option

This patch is for linux-6.1.y and earlier, it has no direct mainline
equivalent.

In order to backport commit d5c8d6e0fa61 ("kbuild: Update assembler
calls to use proper flags and language target") to resolve a separate
issue regarding PowerPC, the problem noticed and fixed by
commit 80a20d2f8288 ("MIPS: Always use -Wa,-msoft-float and eliminate
GAS_HAS_SET_HARDFLOAT") needs to be addressed. Unfortunately, 6.1 and
earlier do not contain commit e4412739472b ("Documentation: raise
minimum supported version of binutils to 2.25"), so it cannot be assumed
that all supported versions of GNU as have support for -msoft-float.

In order to switch from KBUILD_CFLAGS to KBUILD_AFLAGS in as-option
without consequence, move the '-Wa,-msoft-float' check to cc-option,
including '$(cflags-y)' directly to avoid the issue mentioned in
commit 80a20d2f8288 ("MIPS: Always use -Wa,-msoft-float and eliminate
GAS_HAS_SET_HARDFLOAT").

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>