host-cpu-c-abi: Add support for LoongArch64 CPU

[Bruno Haible]

On the LoongArch64 CPU, GCC and clang predefine only __loongarch__,
__loongarch64. GCC predefines also __loongarch_lp64. But it's too hard
to remember that __loongarch64__ is not defined. So, let host-cpu-c-abi
define it.

Btw, this is independent of the floating-point variants of the ABI:

-mabi=lp64d (the default) defines
#define __loongarch_hard_float 1
#define __loongarch_double_float 1

-mabi=lp64f defines
#define __loongarch_hard_float 1
#define __loongarch_single_float 1

-mabi=lp64s defines
#define __loongarch_soft_float 1


2022-01-09  Bruno Haible  <bruno@clisp.org>

        host-cpu-c-abi: Add support for LoongArch64 CPU.
        * m4/host-cpu-c-abi.m4 (gl_HOST_CPU_C_ABI): Define __loongarch64__. (GCC
        and clang predefine only __loongarch__, __loongarch64.)

diff --git a/m4/host-cpu-c-abi.m4 b/m4/host-cpu-c-abi.m4
index 6d1e4578c7..b9223241b4 100644
--- a/m4/host-cpu-c-abi.m4
+++ b/m4/host-cpu-c-abi.m4
@@ -1,4 +1,4 @@
-# host-cpu-c-abi.m4 serial 14
+# host-cpu-c-abi.m4 serial 15
 dnl Copyright (C) 2002-2022 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -382,6 +382,9 @@ EOF
 #ifndef __ia64__
 #undef __ia64__
 #endif
+#ifndef __loongarch64__
+#undef __loongarch64__
+#endif
 #ifndef __m68k__
 #undef __m68k__
 #endif