LoongArch: Drop -ffreestanding from CFLAGS

As explained by Nick in the original issue: the kernel usually does a
good job of providing library helpers that have similar semantics as
their ordinary userspace libc equivalents, but -ffreestanding disables
such libcall optimization and other related features in the compiler,
which can lead to unexpected things such as CONFIG_FORTIFY_SOURCE not
working (!).

As it turns out to be the case, removing the flag does not impact the
LoongArch kernel's normal operation at all; so just remove it to
restore expected libcall semantics globally on this architecture.

Closes: ClangBuiltLinux#1897
Reported-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: WANG Xuerui <git@xen0n.name>

arch/loongarch/Makefile

@@ -83,7 +83,6 @@ KBUILD_CFLAGS_KERNEL		+= -fPIE
 LDFLAGS_vmlinux			+= -static -pie --no-dynamic-linker -z notext
 endif
 
-cflags-y += -ffreestanding
 cflags-y += $(call cc-option, -mno-check-zero-division)
 
 load-y		= 0x9000000000200000