I didn't notice this in the previous commit because apparently GCC2
just links against libroot's versions of them. On GCC5, however,
the version from libroot_build was used even for calls from libroot itself,
which led to infinite loops and then stack overflows.
So instead we must have the "syscall" functions in libroot_build shadow
the real ones by being named differently, which I did by changing their
prefix from "_kern" to "_kernbuild" via preprocessor macros.
Since the build syscalls.h is now substantially different than the non-
build one (and has not been synchronized in nearly a decade anyway),
I've just stripped out all the syscall defns except for the ones actually used
in the build.
Thanks to kallisti5 for helping me debug and test.
1465ec71a0