gdb/riscv: Format CORE_ADDR as a string for printing

gdb/riscv: Format CORE_ADDR as a string for printing

Avoid compiler errors caused by trying to print CORE_ADDR using '%ld'
format, instead convert to a string and print that instead.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_scan_prologue): Use plongest to format
	a signed offset as a string.