RISC-V: Fix selection of pipeline model for sifive-7-series

RISC-V: Fix selection of pipeline model for sifive-7-series

A few of the gcc.target/riscv/mcpu-*.c tests have been failing for a
while now, due to the pipeline model for sifive-7-series not being
selected despite -mtune=sifive-7-series.  The root cause is that the
respective RISCV_TUNE entry points to generic instead.  Fix this.

Fixes 97d1ed67fc6 ("RISC-V: Support --target-help for -mcpu/-mtune")

gcc/ChangeLog:

	* config/riscv/riscv-cores.def (RISCV_TUNE): Update
	sifive-7-series to point to the sifive_7 pipeline description.