Merge bitcoin/bitcoin#23603: build: Fix x86_64 <-> arm64 cross-compiling for macOS

19f3896 build: Fix x86_64 <-> arm64 cross-compiling for macOS (Hennadii Stepanov)

Pull request description:

  Currently, on master (111c3e0), dependencies are built for the build system architecture, not the provided host.

  On Intel-based macOS Big Sur 11.6.1 (20G224):
  ```
  % make -C depends HOST=arm64-apple-darwin20
  % lipo -info depends/arm64-apple-darwin20/lib/libsqlite3.a
  Non-fat file: depends/arm64-apple-darwin20/lib/libsqlite3.a is architecture: x86_64
  ```

  On M1-based macOS Monterey 12.0.1 (21A559) the `boost` package building fails with multiple errors like that:
  ```
  % make -C depends boost HOST=x86_64-apple-darwin19
  ...
  error: option 'cf-protection=return' cannot be specified on this target
  error: option 'cf-protection=branch' cannot be specified on this target
  2 errors generated.
  ```

  This PR allows to cross-compile as follows:
  - on Intel-based macOS Big Sur 11.6.1 (20G224):
  ```
  % make -C depends HOST=arm64-apple-darwin20
  % lipo -info depends/arm64-apple-darwin20/lib/libsqlite3.a
  Non-fat file: depends/arm64-apple-darwin20/lib/libsqlite3.a is architecture: arm64
  % CONFIG_SITE=$PWD/depends/arm64-apple-darwin20/share/config.site ./configure
  % make
  % lipo -info src/qt/bitcoin-qt
  Non-fat file: src/qt/bitcoin-qt is architecture: arm64
  ```

  - on M1-based macOS Monterey 12.0.1 (21A559):
  ```
  % make -C depends HOST=x86_64-apple-darwin19
  % CONFIG_SITE=$PWD/depends/x86_64-apple-darwin19/share/config.site ./configure
  % make
  % lipo -info src/qt/bitcoin-qt
  Non-fat file: src/qt/bitcoin-qt is architecture: x86_64
  ```

  No behavior change for other builder-host pairs.

  This is an alternative to bitcoin/bitcoin#22506.

ACKs for top commit:
  fanquake:
    ACK 19f3896 - this is definitely cleaner than the approach in #22506. I've tested the x86_64 -> arm64 and arm64 -> x86_64 cross-compiles. Going to go-ahead and merge this given it's scoped to building for darwin targets, on darwin. Not sure if this is something we'll actually see used very often.

Tree-SHA512: 67a7fed1db79bc43d41aabb5a676f90417a879e5dcffec1384c288b56ee32ba06ae3819abdc7aca85b81af6e782bcfff5c83a67a455560249fa095a807fc1187

Author: fanquake

depends/builders/darwin.mk

@@ -21,3 +21,8 @@ darwin_NM:=$(shell xcrun -f nm)
 darwin_INSTALL_NAME_TOOL:=$(shell xcrun -f install_name_tool)
 darwin_native_binutils=
 darwin_native_toolchain=
+
+x86_64_darwin_CFLAGS = -arch x86_64
+x86_64_darwin_CXXFLAGS = $(x86_64_darwin_CFLAGS)
+aarch64_darwin_CFLAGS = -arch arm64
+aarch64_darwin_CXXFLAGS = $(aarch64_darwin_CFLAGS)