cmd/gomobile: Use parallel build for frameworks on Apple platforms

[MikeChongCan]

Currently the build for different targets and CPU architectures is sequential. Even for a small project, it usually takes about one minute to build for -target maccatalyst,ios,iossimulator,macos, which is very different from our perception of build speed of Golang.
By adopting this parallel build approach, we can save a lot of time.

Simple visualization (gomobile bind -target maccatalyst,ios,iossimulator,macos ...):

Main Branch

1. Build Mac Catalyst (amd64)
2. Build Mac Catalyst (arm64)
   • Merge into a fat binary
3. Build iOS (arm64)
4. Build iOS Simulator (amd64)
5. Build iOS Simulator (arm64)
   • Merge into a fat binary
6. Build macOS (amd64)
7. Build macOS (arm64)
   • Merge into a fat binary
8. Create xcframework

This PR

1. Build different architectures in parallel
2. Merge different architectures for a single target into a fat binary (this is the limitation of xcodebuild. In theory, we don't need to do so. Reference: https://developer.apple.com/forums/thread/666335?answerId=685927022#685927022)
3. Create xcframework

Quick Benchmark

Local machine comparison with a minimal demo Golang project (on M1 Max):

Sample 1

Official demo: https://github.com/golang/mobile/tree/master/example/bind/hello

# This PR
$ time gomobile-parallel bind -v -target maccatalyst,ios,iossimulator,macos -o /path/to/parallel/Demo.xcframework
gomobile-parallel bind -v -target  -o   29.46s user 18.07s system 303% cpu 15.645 total

# Main Branch
$ time gomobile bind -v -target maccatalyst,ios,iossimulator,macos -o /path/to/sequential/Demo.xcframework
gomobile bind -v -target maccatalyst,ios,iossimulator,macos -o   27.10s user 13.18s system 77% cpu 52.052 total

Sample 2

# This PR
gomobile-parallel bind -target  -o   30.98s user 17.43s system 183% cpu 26.400 total

# Main branch
gomobile bind -target maccatalyst,ios,iossimulator,macos -o   25.80s user 13.17s system 38% cpu 1:42.43 total

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This PR (HEAD: 0572ebc) has been imported to Gerrit for code review.

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[MikeChongCan]

@hyangah gentle ping : )