This enables math module for riscv32 targets #669

Job	Run time
Rustfmt	11s
Clippy	19s
Test (aarch64-unknown-linux-gnu, ubuntu-latest, nightly)	3m 22s
Test (arm-unknown-linux-gnueabi, ubuntu-latest, nightly)	2m 33s
Test (arm-unknown-linux-gnueabihf, ubuntu-latest, nightly)	2m 38s
Test (i586-unknown-linux-gnu, ubuntu-latest, nightly)	2m 24s
Test (i686-unknown-linux-gnu, ubuntu-latest, nightly)	2m 22s
Test (powerpc-unknown-linux-gnu, ubuntu-latest, nightly)	2m 59s
Test (powerpc64-unknown-linux-gnu, ubuntu-latest, nightly)	3m 2s
Test (powerpc64le-unknown-linux-gnu, ubuntu-latest, nightly)	2m 55s
Test (thumbv6m-none-eabi, ubuntu-latest, nightly)	2m 20s
Test (thumbv7em-none-eabi, ubuntu-latest, nightly)	1m 43s
Test (thumbv7em-none-eabihf, ubuntu-latest, nightly)	1m 53s
Test (thumbv7m-none-eabi, ubuntu-latest, nightly)	1m 51s
Test (wasm32-unknown-unknown, ubuntu-latest, nightly)	2m 43s
Test (x86_64-unknown-linux-gnu, ubuntu-latest, nightly)	2m 26s
Test (x86_64-apple-darwin, macos-latest, nightly)	2m 15s
Test (i686-pc-windows-msvc, windows-latest, nightly)	3m 10s
Test (x86_64-pc-windows-msvc, windows-latest, nightly)	2m 59s
Test (i686-pc-windows-gnu, windows-latest, nightly-i686-gnu)	3m 58s
Test (x86_64-pc-windows-gnu, windows-latest, nightly-x86_64-gnu)	3m 42s
	51m 45s

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