BaseTools/GenFw: Enhance GenFw to support PRM GCC build

BaseTools/GenFw: Enhance GenFw to support PRM GCC build

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3802

Since PRM module needs to support export table in PE-COFF, we'll
enhance GenFw tool to support this.

Add one export flag in GenFw tool. If export flag is set:
Step1: Scan ELF symbol table based on PRM module descriptor to get
descriptor offset address;
Step2: Find PRM handlers number and name in COFF file based on the
address from step1;
Step3: Write PRM info such as handler name and export RVA into COFF
export table.

PRM option currently only supports DXE RUNTIME driver and X64 arch.

Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Lixia Huang <lisa.huang@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>