Historically, NetBSD, FreeBSD and OpenBSD have defined the macro ABICALLS in
the preprocessor when -mabicalls is in effect.
Mainline GCC later defined __mips_abicalls when -mabicalls is in effect.
This patch teaches the preprocessor to define these macros when appropriate.
This resolves PR/31694.
Thanks to Sean Bruno for highlighting this issue!
If GCC always defines this one with abicalls (including on the BSDs) I think we should do the same.