RISC-V backend uses outdated names for some macros from riscv.h

[jdmpapin]

• ENCODE_SBTYPE_IMM()
• ENCODE_UJTYPE_IMM()
• VALID_SBTYPE_IMM()
• VALID_UJTYPE_IMM()

Apparently SBTYPE_IMM and UJTYPE_IMM are outdated terminology and have been renamed to BTYPE_IMM and JTYPE_IMM, respectively. Following the instructions for getting the required RISC-V headers now results in files where the old names are not defined. To continue with my build, I had to track down binutils-gdb commit 5a9f5403c75c and copy the headers from its parent.