pw_build: Remove build directory prefix from paths
pw_build: Remove build directory prefix from paths

Since the build directory can be set to anything in gn gen, remap
absolute paths to it to "out" using -ffile-prefix-map.

The build directory is often nested under //, so it should be removed
first. However, GCC and Clang apply -ffile-prefix-map options in
opposite order, so apply this option first and last to handle both
compilers.

Change-Id: Iba135ba23048b7b0bf8b682397ddabb53dee4360
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/52381
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed