pw_build: Add Pigweed specific Bazel platforms

This change adds a set of constraint_settings constraint_values and
platforms specific to pigweed. This is also tested on pw_boot_armv7m and
pw_sys_io_baremetal_stm32f429 packages.

Change-Id: I991aaff7a6208c6954555d4edd91142156a0db5c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/33580
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Akira Baruah <akira.baruah@gmail.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/WORKSPACE b/WORKSPACE
index adcb010..573d3f2 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -69,7 +69,6 @@
     name = "bazel_embedded",
     commit = "89c05fa415218abd2e24fa7016cb7903317d606b",
     remote = "https://github.com/silvergasp/bazel-embedded.git",
-    shallow_since = "1614045367 +0800",
 )
 
 # Instantiate Pigweed configuration for embedded toolchain,
@@ -128,3 +127,5 @@
 )
 
 register_clang_toolchain()
+
+register_execution_platforms("//pw_build/platforms:all")