RISC-V: Make unique SECCAT_SRODATA names start with .srodata (not .sdata2)

RISC-V: Make unique SECCAT_SRODATA names start with .srodata (not .sdata2)

default_unique_section uses ".sdata2" as a prefix for SECCAT_SRODATA
unique sections, but RISC-V uses ".srodata" instead. Override the
TARGET_ASM_UNIQUE_SECTION function to catch this case, allowing the
default to be used for all other sections.

Signed-off-by: Keith Packard <keithp@keithp.com>
	gcc/
	* config/riscv/riscv.c (riscv_unique_section): New.
	(TARGET_ASM_UNIQUE_SECTION): New.