BaseTools/tools_def.txt: Add -march=i586 for IA32 GCC targets

Newer GCC compilers use a default of march higher than i586
for -m32 (IA32 configuration) and this is causing generation of
instructions that are not compatible with all IA32 targets.
Specically Galileo platform support in the QuarkPlatformPkg does
not boot if GCC48 or higher is used.

This is similar to the following checkin that was done to address
this same issue for VS2012 and higher tool chains:

SHA-1: 71028ba2c4c398d70475504e671d048d9003d90f

Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Leroy Leahy <leroy.p.leahy@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>