[RISCV] Pass -mno-relax to assembler when -fno-integrated-as specified

In the past, `clang --target=riscv64-unknown-linux-gnu -mno-relax -c hello.s` will assemble hello.s without relaxation, but `clang --target=riscv64-unknown-linux-gnu -mno-relax -fno-integrated-as -c hello.s` doesn't pass the `-mno-relax` option to assembler, and assemble with relaxation
This patch pass the -mno-relax option to assembler when -fno-integrated-as is specified.

Differential Revision: https://reviews.llvm.org/D120639