configure: Add linux header compile support for LoongArch

When compiling qemu with system KVM mode for LoongArch, header files
in directory linux-headers/asm-loongarch should be used firstly.
Otherwise it fails to find kvm.h on system with old glibc, since
latest kernel header files are not installed.

This patch adds linux_arch definition for LoongArch system so that
header files in directory linux-headers/asm-loongarch can be included.

Fixes: 714b03c ("target/loongarch: Add loongarch kvm into meson build")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240116013952.264474-1-maobibo@loongson.cn>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

configure

@@ -445,6 +445,7 @@ case "$cpu" in
   loongarch*)
     cpu=loongarch64
     host_arch=loongarch64
+    linux_arch=loongarch
     ;;
 
   mips64*)