build: Respect native-*-tools-path when building Wasm stdlib

[kateinoigakukun]

This change allows the Wasm stdlib to be built using the prebuilt Swift toolchain instead of just built from source without building compiler tools. For example, I'm using the following build preset on my local.

skip-build-llvm
skip-build-swift
skip-build-cmark
skip-build-benchmarks
skip-early-swift-driver

build-wasm-stdlib

native-swift-tools-path=/path/to/swift-source/install/usr/bin
native-clang-tools-path=/path/to/swift-source/install/usr/bin
native-llvm-tools-path=/path/to/swift-source/install/usr/bin

[kateinoigakukun]

@swift-ci Please test WebAssembly

[kateinoigakukun]

@swift-ci smoke test

[kateinoigakukun]

@swift-ci Please test WebAssembly

[kateinoigakukun]

@swift-ci smoke test

[kateinoigakukun]

I'd like to merge this within a few days to unblock my threading support works on Wasm. This change only affects wasm build and unmaintained stdlib_S_standalone_minimal_macho_x86_64, so should be safe.

[edymtt]

I would suggest to run the full macOS PR testing (since that will build minimal-stdlib, which uses the code path modified in utils/swift_build_support/swift_build_support/cmake.py), and the standalone stdlib PR testing (so we ensure that we are not regressing jobs using a prebuilt toolchain)

@swift-ci Please test stdlib with toolchain
@swift-ci Please test macOS platform

[kateinoigakukun]

@swift-ci test

[kateinoigakukun]

@swift-ci Please test stdlib with toolchain

[kateinoigakukun]

https://ci.swift.org/job/swift-PR-stdlib-with-toolchain-osx/11/console
The failed tests are exactly the same as the main branch, so the build is not broken.