Add common embedded target triple mappings

Introduces a variety of commonly used tier-2 embedded baremetal target
triple mappings.

Author: armandomontanez

rust/platform/triple_mappings.bzl

@@ -47,19 +47,24 @@ SUPPORTED_T2_PLATFORM_TRIPLES = {
     "aarch64-linux-android": _support(std = True, host_tools = False),
     "aarch64-pc-windows-msvc": _support(std = True, host_tools = True),
     "aarch64-unknown-fuchsia": _support(std = True, host_tools = False),
+    "aarch64-unknown-none": _support(std = True, host_tools = False),
     "aarch64-unknown-uefi": _support(std = True, host_tools = False),
     "arm-unknown-linux-gnueabi": _support(std = True, host_tools = True),
     "armv7-linux-androideabi": _support(std = True, host_tools = False),
     "armv7-unknown-linux-gnueabi": _support(std = True, host_tools = True),
     "i686-linux-android": _support(std = True, host_tools = False),
     "i686-unknown-freebsd": _support(std = True, host_tools = False),
     "powerpc-unknown-linux-gnu": _support(std = True, host_tools = True),
+    "riscv32imac-unknown-none-elf": _support(std = True, host_tools = False),
     "riscv32imc-unknown-none-elf": _support(std = True, host_tools = False),
     "riscv64gc-unknown-linux-gnu": _support(std = True, host_tools = False),
     "riscv64gc-unknown-none-elf": _support(std = True, host_tools = False),
     "s390x-unknown-linux-gnu": _support(std = True, host_tools = True),
     "thumbv7em-none-eabi": _support(std = True, host_tools = False),
+    "thumbv6m-none-eabi": _support(std = False, host_tools = False),
+    "thumbv7m-none-eabi": _support(std = False, host_tools = False),
     "thumbv8m.main-none-eabi": _support(std = True, host_tools = False),
+    "thumbv8m.main-none-eabihf": _support(std = True, host_tools = False),
     "wasm32-unknown-emscripten": _support(std = True, host_tools = False),
     "wasm32-unknown-unknown": _support(std = True, host_tools = False),
     "wasm32-wasip1": _support(std = True, host_tools = False),
@@ -120,6 +125,7 @@ _CPU_ARCH_TO_BUILTIN_PLAT_SUFFIX = {
     "powerpc64": None,
     "powerpc64le": "ppc64le",
     "riscv32": "riscv32",
+    "riscv32imac": "riscv32",
     "riscv32imc": "riscv32",
     "riscv64": "riscv64",
     "riscv64gc": "riscv64",