[cmake] Add LoongArch to config.guess

Necessary for native builds on LoongArch. Only Linux is added for now,
matching upstream config.git changes.

Unfortunately, the upstream config.guess is GPLv3+ since 2012, and
LLVM is without autoconf support for a long time, so the script's
licensing exception no longer works and a bump could not be trivially
done.

This is the same as the [upstream commit adding LoongArch](https://git.savannah.gnu.org/gitweb/?p=config.git;a=commitdiff;h=c8ddc8472f8efcadafc1ef53ca1d863415fddd5f),
but rewritten to match the 2011 style that our script is holding on
to.

Differential Revision: https://reviews.llvm.org/D130820

llvm/cmake/config.guess

@@ -922,6 +922,30 @@ EOF
     ia64:Linux:*:*)
 	echo ${UNAME_MACHINE}-unknown-linux-gnu
 	exit ;;
+    loongarch32:Linux:*:* | loongarch64:Linux:*:* | loongarchx32:Linux:*:*)
+	LIBC=gnu
+	eval $set_cc_for_build
+	# Do not check for __GLIBC__ because uclibc defines it too
+	sed 's/^	//' << EOF >$dummy.c
+	#include <features.h>
+	#if defined(__UCLIBC__)
+	LIBC=uclibc
+	#elif defined(__dietlibc__)
+	LIBC=dietlibc
+	#endif
+EOF
+	eval `$CC_FOR_BUILD -E $dummy.c 2>/dev/null | grep '^LIBC'`
+
+	# There is no features test macro for musl
+	# Follow the GNU's config.guess approach of
+	# checking the output of ldd
+	if command -v ldd >/dev/null && \
+		ldd --version 2>&1 | grep -q ^musl; then
+	    LIBC=musl
+	fi
+
+	echo "${UNAME_MACHINE}-unknown-linux-${LIBC}"
+	exit ;;
     m32r*:Linux:*:*)
 	echo ${UNAME_MACHINE}-unknown-linux-gnu
 	exit ;;