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

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

This is a port of commit 4fe4a6374c4db9ae2b ("MIPS: Only fiddle with
CHECKFLAGS if `need-compiler'") to LoongArch.

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

Starting from commit 805b2e1d427aab4 ("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.

It doesn't cause problems on LoongArch now. But as a guard we replace
the check for CONFIG_LOONGARCH with one for `need-compiler' instead, so
as to prevent the compiler from being ever called for CHECKFLAGS when
not needed.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>