system.h: add support for IBM xlc C compiler

Added a section to system.h guarded with __xlc__ for the IBM xml C
compiler. Before this change the section titled 'generic "safe guess" on
old 32 bit style' was used, which resulted in a wrong definition of
CURL_TYPEOF_CURL_SOCKLEN_T, and for 64-bit also CURL_TYPEOF_CURL_OFF_T
was wrong.

Compilation warnings fixed with this change:

  CC       libcurl_la-ftp.lo
"ftp.c", line 290.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 293.48: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1070.49: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1154.53: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1187.51: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
  CC       libcurl_la-connect.lo
"connect.c", line 448.56: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 516.66: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 687.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 696.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
  CC       libcurl_la-tftp.lo
"tftp.c", line 1115.33: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.

Closes #2637