loongarch64: add initial loongarch support

This commit adds architecture support for LoongArch.

The toolchain currently only supports the 64-bit target
loongarch64-unknown-linux-gnu.

It has been tested to build with GCC 12.1, GDB 12.1, Glibc 2.36, Linux
5.19 and Binutils 2.39 as of Aug 2022.

Signed-off-by: Jiajie Chen <c@jia.je>

config/arch/loongarch.in

@@ -0,0 +1,19 @@
+# LoongArch specific config options
+
+## no-package
+## depends on EXPERIMENTAL
+##
+## select ARCH_SUPPORTS_64
+## select ARCH_DEFAULT_64
+## select ARCH_SUPPORTS_BOTH_MMU
+## select ARCH_SUPPORTS_WITH_ABI
+## select ARCH_SUPPORTS_WITH_ARCH
+## select ARCH_SUPPORTS_WITH_TUNE
+## select GCC_REQUIRE_12_or_later
+## select BINUTILS_REQUIRE_2_39_or_later
+## select LINUX_REQUIRE_5_19_or_later
+## select GDB_REQUIRE_12_1_or_later if DEBUG_GDB
+## select GLIBC_REQUIRE_2_36_or_later if LIBC_GLIBC
+
+## help The LoongArch architecture, as defined by:
+## help     https://loongson.github.io/LoongArch-Documentation/README-EN.html

scripts/build/arch/loongarch.sh

@@ -0,0 +1,5 @@
+# Compute LoongArch-specific values
+
+CT_DoArchTupleValues() {
+    CT_TARGET_ARCH="loongarch${CT_ARCH_BITNESS}"
+}