[RISCV][NFC] s/riscv32-linux-unknown-elf/riscv32-unknown-linux-gnu in...

[RISCV][NFC] s/riscv32-linux-unknown-elf/riscv32-unknown-linux-gnu in test/Driver/riscv32-toolchain.c

riscv32-linux-unknown-elf was a weird thing to test for as it doesn't match
the triple used in any common RISC-V toolchain distributions (e.g.
riscv-gnu-toolchain scripts produce riscv{32,64}-unknown-linux-gnu).

llvm-svn: 357693