devel/freebsd-sysroot: Add -mno-relax to the riscv64 wrapper scripts

$ FREEBSD_SYSROOT_CC=clang13 /usr/local/freebsd-sysroot/riscv64/bin/cc test.c
ld: error: test.c:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax

On riscv64 base clang defaults to -mno-relax (per D25210). This
patch has not been applied to ports LLVM. In the wrapper scripts
pass -mno-relax explicitly to make it work as expected when
FREEBSD_SYSROOT_CC is set to ports clang.