RISC-V: Add target machine headers as a dependency for riscv-sr.o

RISC-V: Add target machine headers as a dependency for riscv-sr.o

Make riscv-sr.o depend on target machine headers, removing spurious test
failures:

FAIL: gcc.target/riscv/save-restore-3.c scan-assembler-not call[ \t]*t0,__riscv_save_0
FAIL: gcc.target/riscv/save-restore-3.c scan-assembler-not tail[ \t]*__riscv_restore_0
FAIL: gcc.target/riscv/save-restore-3.c scan-assembler tail[ \t]*foo
FAIL: gcc.target/riscv/save-restore-6.c scan-assembler-not call[ \t]*t0,__riscv_save_0
FAIL: gcc.target/riscv/save-restore-6.c scan-assembler-not tail[ \t]*__riscv_restore_0
FAIL: gcc.target/riscv/save-restore-6.c scan-assembler tail[ \t]*other_func

if the definitions of UNSPECs are locally changed and GCC rebuilt from a
dirty tree.

	gcc/
	* config/riscv/t-riscv (riscv-sr.o): Add $(TM_H) dependency.