No fastcall attribute in POWER platform
Compiling the development branch of pandas fail in AIX. 

pandas/_libs/src/ujson/lib/ultrajsonenc.c:397:1: error: ‘fastcall’ attribute directive ignored [-Werror=attributes]
 Buffer_AppendShortHexUnchecked(char *outputOffset, unsigned short value) {
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/src/ujson/lib/ultrajsonenc.c:726:59: error: ‘fastcall’ attribute directive ignored [-Werror=attributes]
                                                           char *end) {
                                                           ^~~~
cc1: all warnings being treated as errors


The fastcall attribute is not available in POWER platform also. 

