Merge bitcoin/bitcoin#23817: build: Fix regression introduced in PR23603

d3b0f82a43 build: Fix regression introduced in PR23603 (Hennadii Stepanov)

Pull request description:

  It appears 7629efcc2c from bitcoin/bitcoin#23603 introduced a regression in build tool flag evaluation.

  On macOS system:
  - pre-PR23603 master (ae017b8160):
  ```
  % make -C depends print-darwin_CXXFLAGS
  darwin_CXXFLAGS=-pipe
  % make -C depends print-host_CXXFLAGS
  host_CXXFLAGS=-pipe
  ```
  - the current master (369978686e):
  ```
  % make -C depends print-darwin_CXXFLAGS
  darwin_CXXFLAGS=-pipe
  % make -C depends print-host_CXXFLAGS
  host_CXXFLAGS=-arch x86_64
  ```

  It's obvious a flag being set in `depends/hosts/darwin.mk`, i.e., `-pipe`, is lost.

  With this PR:
  ```
  % make -C depends print-darwin_CXXFLAGS
  darwin_CXXFLAGS=-pipe
  % make -C depends print-host_CXXFLAGS
  host_CXXFLAGS=-pipe -arch x86_64
  ```

ACKs for top commit:
  fanquake:
    ACK d3b0f82a43

Tree-SHA512: 643099ce6858475ac9f3a4dfa72a4e493fec6fdd7042ae0f0d5fe44c5cd175e4eda63cb39fc46ac1501cadcd3466507ec88d9089235e005fe43ea7ab47ce37c1

depends/builders/darwin.mk

@@ -22,7 +22,7 @@ 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)
+x86_64_darwin_CFLAGS += -arch x86_64
+x86_64_darwin_CXXFLAGS += -arch x86_64
+aarch64_darwin_CFLAGS += -arch arm64
+aarch64_darwin_CXXFLAGS += -arch arm64