BaseTools GenFw: Support CLANG8ELF with conversion ELF to PE/COFF image
    
    CLANG8ELF tool chain generated ELF image with the different attributes
    in section. Update GenFw to handle them.
    1. .text section with writable attribute (support)
    2. .reloc section has the symbol for *ABS* (skip)
    
    Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
    Reviewed-by: Feng Bob C <bob.c.feng@intel.com>