LoongArch: Replace -ffreestanding with finer-grained -fno-builtin's

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 (!).

However, due to the desire for better control over unaligned accesses
with respect to CONFIG_ARCH_STRICT_ALIGN, and also for avoiding the
GCC bug https://gcc.gnu.org/PR109465, we do want to still disable
optimizations for the memory libcalls (memcpy, memmove and memset for
now). Use finer-grained -fno-builtin-* toggles to achieve this without
losing source fortification and other libcall optimizations.

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

Author: xen0n

arch/loongarch/Makefile

@@ -83,8 +83,8 @@ 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)
+cflags-y += -fno-builtin-memcpy -fno-builtin-memmove -fno-builtin-memset
 
 load-y		= 0x9000000000200000
 bootvars-y	= VMLINUX_LOAD_ADDRESS=$(load-y)