ICU-22838 Add WebAssembly/WASI cross-compilation support

[kateinoigakukun]

With this patch, we can cross-compile ICU4C with wasi-sdk as follows:

$ mkdir tools
$ mkdir cross
$ cd tools
$ ../icu/icu4c/source/configure && make
$ cd ../cross
$ ../../icu/icu4c/source/configure --host=wasm32-unknown-wasi \
            --with-cross-build="$PWD/../tools" \
            --enable-static --disable-shared \
            --disable-tools --disable-tests \
            --disable-samples --disable-extras \
            --with-data-packaging=static \
            --prefix / \
            CC="$WASI_SDK_PATH/bin/clang" CXX="$WASI_SDK_PATH/bin/clang++" \
            AR="$WASI_SDK_PATH/bin/llvm-ar" RANLIB="$WASI_SDK_PATH/bin/llvm-ranlib" \
            CFLAGS="--sysroot $WASI_SDK_PATH/share/wasi-sysroot -D_WASI_EMULATED_SIGNAL" \
            CXXFLAGS="-fno-exceptions --sysroot $WASI_SDK_PATH/share/wasi-sysroot -D_WASI_EMULATED_SIGNAL"
$ make PKGDATA_OPTS="--without-assembly -O $PWD/data/icupkg.inc" -j16

Patch overview

• Add config/mh-wasi and update configure script.

  WebAssembly/WASI does not support threads and dynamic linking, so
  the new mh-wasi file disables those features in ICU.
  The new config file mh-wasi is used when the target matches *-wasi*,
  which includes wasm32-unknown-wasi, wasm32-unknown-wasip1, and etc.

• Teach platform.h not to define U_TZSET, U_TIMEZONE, and U_TZNAME for WASI

  WASI does not define timezone-related functionalities, and wasi-libc
  does not provide tzset, timezone, and tzname. This change teaches
  platform.h not to define U_TZSET, U_TIMEZONE, and U_TZNAME for WASI.

• Add U_HAVE_ATOMICS and support platforms without atomics like WASI

  This change adds a new macro U_HAVE_ATOMICS to ICU. This macro is
  always defined to 1 except for platforms that do not support atomic
  operations like WASI. Due to the lack of threading support in WASI, the
  wasi-sdk does not provide std::atomic, std::mutex, and related
  types, so the new macro is used to disable the use of these types.

After #3066 and this are merged, I'd like to add CI check for WASI cross-compilation

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22838
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/common/putilimp.h is different
• icu4c/source/common/unicode/platform.h is now changed in the branch
• icu4c/source/i18n/unicode/numberrangeformatter.h is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/i18n/number_mapper.h is different
• icu4c/source/i18n/numrange_fluent.cpp is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[markusicu]

@srl295 @sffc could you help review these changes?