gdb: Work around lack of native macOS/arm64 target support

We don't actually care about having support for macOS targets, except
that it means the build system automatically gives us unprefixed file
names. However, we can't do that on macOS/arm64, so use a macOS/amd64
triple as the closest thing and manually symlink to hide the ugliness
somewhat from the user.

pycheribuild/projects/cross/gdb.py

@@ -27,6 +27,7 @@
 # OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 # SUCH DAMAGE.
 #
+import os
 import shutil
 
 from .crosscompileproject import (
@@ -146,6 +147,14 @@ def setup(self) -> None:
         self.cross_warning_flags.append("-Wno-error=incompatible-pointer-types")
         self.configure_args.append("--enable-targets=all")
         if self.compiling_for_host():
+            # GDB can't target macOS/arm64 so use a macOS/amd64 triple
+            if self.target_info.is_macos() and self.crosscompile_target.is_aarch64():
+                x86_target = "x86_64-apple-darwin" + os.uname().release
+                self.configure_args.append("--target=" + x86_target)
+                self.native_target_prefix = x86_target + "-"
+            else:
+                self.native_target_prefix = None
+
             if self.target_info.is_freebsd():
                 self.LDFLAGS.append(f"-L{self.target_info.localbase}/lib")  # Expat/GMP are in $LOCALBASE
             self.configure_args.append("--with-expat")
@@ -217,6 +226,7 @@ def install(self, **kwargs):
         # Install the binutils prefixed with g (like homebrew does it on MacOS)
         # objdump is useful for cases where CHERI llvm-objdump doesn't print sensible source lines
         # Also install most of the other tools in case they work better than elftoolchain
+        # Also symlink macOS/amd64 triple prefixed gdb files to their unprefixed names on macOS/arm64
         if self.compiling_for_host():
             binutils = ("objdump", "objcopy", "addr2line", "readelf", "ar", "ranlib", "size", "strings")
             bindir = self.install_dir / "bin"
@@ -227,6 +237,14 @@ def install(self, **kwargs):
             self.install_file(self.build_dir / "binutils/nm-new", bindir / "gnm")
             self.install_file(self.build_dir / "binutils/strip-new", bindir / "gstrip")
 
+            if self.native_target_prefix is not None:
+                gdbfiles = ("man/man5/gdbinit.5", "man/man1/gdbserver.1", "man/man1/gdb.1", "man/man1/gdb-add-index.1",
+                            "bin/gdb", "bin/gdb-add-index")
+                for file in gdbfiles:
+                    dst = self.install_dir / file
+                    src = dst.parent / (self.native_target_prefix + dst.name)
+                    self.create_symlink(src, dst)
+
 
 class BuildUpstreamGDB(BuildGDBBase):
     repository = GitRepository("git://sourceware.org/git/binutils-gdb.git")