UefiCpuPkg: LocalApicLib: Add API to set SoftwareEnable bit

UefiCpuPkg: LocalApicLib: Add API to set SoftwareEnable bit

The LocalApicLib does not provide a function to manage the state of the
Local APIC SoftwareEnable bit in the Spurious Vector register.  There
are cases where this bit needs to be managed without side effects to.
other Local APIC registers.  One use case is in the DebugAgent in the
SourceLevelDebugPkg.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18711 6f19259b-4bc3-4df7-8a09-765794883524