Use a BufWriter in emit_module to reduce syscall overhead #3656

Job	Run time
rustfmt	11s
dist (ubuntu-20.04, x86_64-unknown-linux-gnu)	1m 41s
test (ubuntu-latest, x86_64-unknown-linux-gnu)	7m 13s
dist (macos-latest, x86_64-apple-darwin)	3m 58s
bench	7m 23s
dist (macos-latest, aarch64-apple-darwin)	1m 33s
test (macos-latest, x86_64-apple-darwin)	21m 17s
dist (ubuntu-latest, x86_64-pc-windows-gnu)	1m 43s
test (ubuntu-latest, x86_64-pc-windows-gnu, gcc-mingw-w64-x86-64 wine-stable)	6m 24s
dist (windows-latest, x86_64-pc-windows-msvc)	2m 32s
test_llvm	3m 37s
test (ubuntu-latest, aarch64-unknown-linux-gnu, gcc-aarch64-linux-gnu qemu-user)	4m 7s
dist (windows-latest, x86_64-pc-windows-gnu)	3m 11s
test (macos-latest, aarch64-apple-darwin)	6m 3s
test (ubuntu-latest, s390x-unknown-linux-gnu, gcc-s390x-linux-gnu qemu-user)	3m 56s
test (ubuntu-latest, riscv64gc-unknown-linux-gnu, gcc-riscv64-linux-gnu qemu-user)	4m 0s
test (windows-latest, x86_64-pc-windows-msvc)	11m 18s
test (windows-latest, x86_64-pc-windows-gnu)	8m 23s
release	0s
	1h 38m 30s

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