pw_toolchain: Register Cortex-M7 toolchain

We apparently added this toolchain in http://pwrev.dev/183160, but
forgot to register it! This is pretty confusing, because we register all
the other Cortex-M toolchains.

See
https://discord.com/channels/691686718377558037/735952592894689350/1269916490614964224
for an example of someone affected by this.

Change-Id: I24339b857ff666f4be4d9368b8bd40b9b79e4297
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/227598
Lint: Lint 🤖 <[email protected]>
Presubmit-Verified: CQ Bot Account <[email protected]>
Reviewed-by: Armando Montanez <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Pigweed-Auto-Submit: Ted Pudlik <[email protected]>

pw_toolchain/register_toolchains.bzl

@@ -72,6 +72,7 @@ def register_pigweed_cxx_toolchains():
         "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m3",
         "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m4",
         "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m4+nofp",
+        "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m7",
         "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m33",
         "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m33+nofp",
         "//pw_toolchain/host_clang:host_cc_toolchain_linux",