8347033: [mobile] add ios-specific build docs

Reviewed-by: jvos, ihse

Author: abhinayagarwal

README.md

@@ -10,3 +10,48 @@ or either of these files:
 See <https://openjdk.org/> for more information about the OpenJDK
 Community and the JDK and see <https://bugs.openjdk.org> for JDK issue
 tracking.
+
+## Build static image for iOS
+
+> [!NOTE]
+> Follow the instructions in [building.md](building.md) to make sure
+> you can build the JDK targeting macOS before trying to build for iOS
+
+### Pre-requisites
+Following are the prerequisites to build JDK on Mac targeting iOS:
+1. Download and unzip pre-built JDK24 to be used as boot JDK
+2. Download the [support zip](https://download2.gluonhq.com/mobile/mobile-support-20250106.zip) which contains an ios build for libffi and cups. Unzip it in the root directory of the project.
+3. Install `autoconf` on mac via homebrew: `brew install autoconf`
+
+### Clone the mobile repository
+Clone the openjdk/mobile repository. The following command can be used:
+
+```
+git clone [email protected]:openjdk/mobile.git
+```
+
+### Configure
+Modify the configure script below so that it has the correct paths to your JDK24 directory,
+the unzipped 'support' directory, and the correct location of the iPhoneOS.platform.
+
+```
+sh configure \
+--with-conf-name=zero-ios-aarch64 \
+--disable-warnings-as-errors \
+--openjdk-target=aarch64-macos-ios \
+--with-boot-jdk=/Users/abhinay/.sdkman/candidates/java/24.ea.29-open \
+--with-jvm-variants=zero \
+--with-libffi-include=<support-dir-absolute-path>/libffi/include \
+--with-libffi-lib=<support-dir-absolute-path>/libffi/libs \
+--with-cups-include=<support-dir-absolute-path>/cups-2.3.6
+--with-sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk \
+```
+
+### Build static image
+Execute the `make` command:
+
+```
+make CONF=zero-ios-aarch64 static-libs-image
+```
+
+Once the build is successful, it should have created a directory in `build/zero-ios-aarch64/images/static-libs/lib/zero` with a file `libjvm.a`.