[roll third_party/pigweed] pw_toolchain_bazel: Introduce pw_cc_flag_set and pw_cc_flag_group

As the first step of implementing toolchains as proposed in SEED-0113,
this CL introduces pw_cc_flag_set and pw_cc_flag_group. These will be
the core building blocks for specifying flags that will be bound to
toolchains.

As an illustrative example, moves the warning "features" to be
pw_cc_flag_set rules instead.

Original-Bug: b/309533028
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/179932

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 6372cdef44d0338..3c3dc003812d05b
Roller-URL: https://ci.chromium.org/b/8764395149860555905
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I14c6cd3d323d2851c48187a2a439fba53e8ca8a1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/180809
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 6372cde..3c3dc00 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 6372cdef44d0338d47108c90c55ad38debcf6c30
+Subproject commit 3c3dc003812d05bb31a47b93ff1df5666ad3db7f