MIPS: Only fiddle with CHECKFLAGS if `need-compiler'

We have originally guarded fiddling with CHECKFLAGS in our arch Makefile
by checking for the CONFIG_MIPS variable, not set for targets such as
`distclean', etc. that neither include `.config' nor use the compiler.

Starting from commit 805b2e1d

 ("kbuild: include Makefile.compiler
only when compiler is needed") we have had a generic `need-compiler'
variable explicitly telling us if the compiler will be used and thus its
capabilities need to be checked and expressed in the form of compilation
flags.  If this variable is not set, then `make' functions such as
`cc-option' are undefined, causing all kinds of weirdness to happen if
we expect specific results to be returned, most recently:

cc1: error: '-mloongson-mmi' must be used with '-mhard-float'

messages with configurations such as `fuloong2e_defconfig' and the
`modules_install' target, which does include `.config' and yet does not
use the compiler.

Replace the check for CONFIG_MIPS with one for `need-compiler' instead,
so as to prevent the compiler from being ever called for CHECKFLAGS when
not needed.

Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Closes: https://lore.kernel.org/r/85031c0c-d981-031e-8a50-bc4fad2ddcd8@collabora.com/


Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Fixes: 805b2e1d

 ("kbuild: include Makefile.compiler only when compiler is needed")
Cc: stable@vger.kernel.org # v5.13+
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>