BaseTools GenFw: Keep read only alloc section as text when convert ELF

BaseTools GenFw: Keep read only alloc section as text when convert ELF

This is the fix of the regression issue at c6b872c6.
Based on ELF spec, readonly alloc section is .rodata section. It is used.
This fix is to add back original check logic for ELF section. Now,
the readonly alloc section and execute alloc section are regarded as .text.

Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>