Bug 1380118 - aom: add x86-win32-gcc config. r=froydnj,tjr

This resolves some issues with building on mingw.

Thanks to Tom Ritter for help developing this patch.

MozReview-Commit-ID: BmJ5TbQAq17

--HG--
extra : rebase_source : 98791b8aab139601f3df4c457a42590a31eeeaea

media/libaom/generate_sources_mozbuild.sh

@@ -198,6 +198,7 @@ gen_config_files linux/ia32 "--target=x86-linux-gcc ${all_platforms} ${x86_platf
 gen_config_files mac/x64 "--target=x86_64-darwin9-gcc ${all_platforms} ${x86_platforms}"
 gen_config_files win/x64 "--target=x86_64-win64-vs14 ${all_platforms} ${x86_platforms}"
 gen_config_files win/ia32 "--target=x86-win32-vs14 ${all_platforms} ${x86_platforms}"
+gen_config_files win/mingw32 "--target=x86-win32-gcc ${all_platforms} ${x86_platforms}"
 
 gen_config_files linux/arm "--target=armv7-linux-gcc ${all_platforms} ${arm_platforms}"
 
@@ -218,6 +219,7 @@ gen_rtcd_header linux/ia32 x86
 gen_rtcd_header mac/x64 x86_64
 gen_rtcd_header win/x64 x86_64
 gen_rtcd_header win/ia32 x86
+gen_rtcd_header win/mingw32 x86
 
 gen_rtcd_header linux/arm armv7