riscv_common: explicitly mark handle_trap as used

The handle_trap function is used internally by the trap_entry
implementation from the same file. However, the trap_entry
implementation calls handle_trap from inline assembly. This makes it
difficult for the compiler to infer that the handle_trap function is
used at all. This causes issues when LTO is enabled.

Without this patch compiling any RISC-V RIOT code with `LTO=1` causes
the following linker error:

	/home/soeren/src/RIOT/cpu/riscv_common/irq_arch.c:134: undefined reference to `handle_trap'
	/tmp/hello-world.elf.Nngidp.ltrans0.ltrans.o:cpu/riscv_common/irq_arch.c:134:(.text.trap_entry+0x34):
	  relocation truncated to fit: R_RISCV_GPREL_I against undefined symbol `handle_trap'

This commit fixes LTO support for RISC-V.

While at it, also mark the function as static as it is only used by the
trap_entry function from the same compilation unit.

cpu/riscv_common/irq_arch.c

@@ -70,7 +70,7 @@ void riscv_irq_init(void)
 /**
  * @brief Global trap and interrupt handler
  */
-void handle_trap(uint32_t mcause)
+static void __attribute((used)) handle_trap(uint32_t mcause)
 {
     /*  Tell RIOT to set sched_context_switch_request instead of
      *  calling thread_yield(). */