Add explicit --sdk argument to xcrun calls

Xcode 11 (and possibly earlier versions) has a bug where, when you don't
pass `-sdk macosx`, it can either pick the global
`/Library/Developer/CommandLineTools` SDK path, or the SDK path bundled
within Xcode itself. This seems to depend on if you have other versions
of Xcode installed, and which versions of the command line tools you
have installed. Passing `-sdk macosx` seems to always force it to pick
the Xcode bundled version, which is what we prefer. This fixes issues
with remote cache misses caused by these tools not being the same
because of this difference. At least 2 apple bugs have been filed
about this FB7147521 FB7253366

You can see which path it's picking by running:

```
env -i xcrun -l -n clang
```

Closes #9371.

PiperOrigin-RevId: 268906772

scripts/bootstrap/compile.sh

diff --git a/scripts/bootstrap/compile.sh b/scripts/bootstrap/compile.sh
index 06a9854..dcc98b4 100755
--- a/scripts/bootstrap/compile.sh
+++ b/scripts/bootstrap/compile.sh
@@ -398,7 +398,7 @@
 # TODO(b/28965185): Remove when xcode-locator is no longer required in embedded_binaries.
 log "Compiling xcode-locator..."
 if [[ $PLATFORM == "darwin" ]]; then
-  run /usr/bin/xcrun clang -fobjc-arc -framework CoreServices -framework Foundation -o ${ARCHIVE_DIR}/_embedded_binaries/xcode-locator tools/osx/xcode_locator.m
+  run /usr/bin/xcrun --sdk macosx clang -fobjc-arc -framework CoreServices -framework Foundation -o ${ARCHIVE_DIR}/_embedded_binaries/xcode-locator tools/osx/xcode_locator.m
 else
   cp tools/osx/xcode_locator_stub.sh ${ARCHIVE_DIR}/_embedded_binaries/xcode-locator
 fi

src/tools/xcode/realpath/BUILD

diff --git a/src/tools/xcode/realpath/BUILD b/src/tools/xcode/realpath/BUILD
index 8642f9c..14d1745 100644
--- a/src/tools/xcode/realpath/BUILD
+++ b/src/tools/xcode/realpath/BUILD
@@ -5,7 +5,7 @@
     name = "realpath_genrule",
     srcs = ["realpath.c"],
     outs = ["realpath"],
-    cmd = "/usr/bin/xcrun clang -o $@ $<",
+    cmd = "/usr/bin/xcrun --sdk macosx clang -o $@ $<",
     output_to_bindir = 1,
     visibility = ["//visibility:public"],
 )

tools/cpp/osx_cc_configure.bzl

diff --git a/tools/cpp/osx_cc_configure.bzl b/tools/cpp/osx_cc_configure.bzl
index 9434c16..7202d49 100644
--- a/tools/cpp/osx_cc_configure.bzl
+++ b/tools/cpp/osx_cc_configure.bzl
@@ -108,6 +108,8 @@
             "env",
             "-i",
             "xcrun",
+            "--sdk",
+            "macosx",
             "clang",
             "-std=c++11",
             "-lc++",

tools/osx/BUILD

diff --git a/tools/osx/BUILD b/tools/osx/BUILD
index b1326e2..b3568dc 100644
--- a/tools/osx/BUILD
+++ b/tools/osx/BUILD
@@ -18,7 +18,7 @@
 ])
 
 DARWIN_XCODE_LOCATOR_COMPILE_COMMAND = """
-  /usr/bin/xcrun clang -fobjc-arc -framework CoreServices \
+  /usr/bin/xcrun --sdk macosx clang -fobjc-arc -framework CoreServices \
       -framework Foundation -o $@ $<
 """
 

tools/osx/xcode_configure.bzl

diff --git a/tools/osx/xcode_configure.bzl b/tools/osx/xcode_configure.bzl
index 6136bcd..509c165 100644
--- a/tools/osx/xcode_configure.bzl
+++ b/tools/osx/xcode_configure.bzl
@@ -117,6 +117,8 @@
         "env",
         "-i",
         "xcrun",
+        "--sdk",
+        "macosx",
         "clang",
         "-fobjc-arc",
         "-framework",