MdePkg: BaseCpuLib: Fix RISCV CpuSleep symbol name.

CpuSleep, not _CpuSleep.

Cc: Daniel Schaefer <git@danielschaefer.me>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Andrei Warkentin <andrei.warkentin@intel.com>

MdePkg/Library/BaseCpuLib/RiscV/Cpu.S

@@ -10,9 +10,9 @@
 .align 3
 .section .text
 
-.global ASM_PFX(_CpuSleep)
+.global ASM_PFX(CpuSleep)
 
-ASM_PFX(_CpuSleep):
+ASM_PFX(CpuSleep):
     wfi
     ret