Use Android NDK 27b and update 32-bit ARM Android API level to 21.

The version of the Android NDK used in GitHub Actions,
/usr/local/lib/android/sdk/ndk/27.1.12297006, doesn't support
SDK version 19 (or 20) any more.

mk/cargo.sh

@@ -39,7 +39,7 @@ qemu_x86_64="qemu-x86_64"
 # be needed to compile the build script, or to compile for other targets.
 if [ -n "${ANDROID_HOME-}" ]; then
   # Keep the next line in sync with the corresponding line in install-build-tools.sh.
-  ndk_version=25.2.9519653
+  ndk_version=27.1.12297006
   ANDROID_NDK_ROOT=${ANDROID_NDK_ROOT:-${ANDROID_HOME}/ndk/$ndk_version}
 fi
 if [ -n "${ANDROID_NDK_ROOT-}" ]; then
@@ -67,7 +67,7 @@ case $target in
    aarch64-linux-android)
     export CC_aarch64_linux_android=$android_tools/aarch64-linux-android21-clang
     export AR_aarch64_linux_android=$android_tools/llvm-ar
-    export CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=$android_tools/aarch64-linux-android21-clang
+    export CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=$CC_aarch64_linux_android
     ;;
   aarch64-unknown-linux-gnu)
     use_clang=1
@@ -94,9 +94,13 @@ case $target in
     export CARGO_TARGET_ARM_UNKNOWN_LINUX_GNUEABIHF_RUNNER="$qemu_arm_gnueabihf"
     ;;
   armv7-linux-androideabi)
-    export CC_armv7_linux_androideabi=$android_tools/armv7a-linux-androideabi19-clang
+    # https://github.com/android/ndk/wiki/Changelog-r26#announcements says API
+    # level 21 is the minimum supported as of NDK 26, even though we'd like to
+    # support API level 19. Rust 1.82 is doing the same; see
+    # https://github.com/rust-lang/rust/commit/6ef11b81c2c02c3c4b7556d1991a98572fe9af87.
+    export CC_armv7_linux_androideabi=$android_tools/armv7a-linux-androideabi21-clang
     export AR_armv7_linux_androideabi=$android_tools/llvm-ar
-    export CARGO_TARGET_ARMV7_LINUX_ANDROIDEABI_LINKER=$android_tools/armv7a-linux-androideabi19-clang
+    export CARGO_TARGET_ARMV7_LINUX_ANDROIDEABI_LINKER=$CC_armv7_linux_androideabi
     ;;
   armv7-unknown-linux-gnueabihf)
     export CC_armv7_unknown_linux_gnueabihf=arm-linux-gnueabihf-gcc

mk/install-build-tools.sh

@@ -41,17 +41,18 @@ case ${target-} in
   # https://blog.rust-lang.org/2023/01/09/android-ndk-update-r25.html says
   # "Going forward the Android platform will target the most recent LTS NDK,
   # allowing Rust developers to access platform features sooner. These updates
-  # should occur yearly and will be announced in release notes." Assume that
-  # means that we should always prefer to be using the latest 25.x.y version of
-  # the NDK until the Rust project announces that we should use a higher major
-  # version number.
+  # should occur yearly and will be announced in release notes."
   #
-  # TODO: This should probably be implemented as a map of Rust toolchain version
-  # to NDK version; e.g. our MSRV might (only) support an older NDK than the
-  # latest stable Rust toolchain.
+  # https://github.com/actions/runner-images/issues/10614 indicates that GitHub
+  # actions doesn't intend to keep unsupported versions around, so in general
+  # we'll end up only supporting the latest NDK even for MSRV builds.
+  #
+  # https://developer.android.com/ndk/guides/other_build_systems explains how
+  # to set the API level.
   #
   # Keep the following line in sync with the corresponding line in cargo.sh.
-  ndk_version=25.2.9519653
+  #
+  ndk_version=27.1.12297006
 
   mkdir -p "${ANDROID_HOME}/licenses"
   android_license_file="${ANDROID_HOME}/licenses/android-sdk-license"