Merge bitcoin/bitcoin#22070: build: don't use cf-protection when targeting arm-apple-darwin

3caedb4 build: don't use cf-protection when targeting arm-apple-darwin (fanquake)

Pull request description:

  After two reports on IRC of issues building depends on an Apple M1 machine, this option (obviously) can't be used when targeting `arm-apple-darwin`.  For now, just use it for `x86_64-apple-darwin`.

  ```bash
  Apple clang version 12.0.5 (clang-1205.0.22.9)
  Target: x86_64-apple-darwin20.4.0

  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.
  ```

ACKs for top commit:
  promag:
    Tested ACK 3caedb4.

Tree-SHA512: 8763a5b94000016b0c2f0438e66002fdfcd2cbafd9d2d2acc1972f0c6f707e820186711dbd9d3f72673c179718da75588acb4732f8d84b85f0c1dfc862b6e944

Author: fanquake

depends/packages/boost.mk

@@ -26,7 +26,7 @@ $(package)_config_libraries=filesystem,system,test
 $(package)_cxxflags=-std=c++17 -fvisibility=hidden
 $(package)_cxxflags_linux=-fPIC
 $(package)_cxxflags_android=-fPIC
-$(package)_cxxflags_darwin=-fcf-protection=full
+$(package)_cxxflags_x86_64_darwin=-fcf-protection=full
 endef
 
 define $(package)_preprocess_cmds