lttng-modules: fix build for non-x86

Since commit 644d6e9c2807041db2b4fd9c3350d4fa243060b1 "Remove sys_
prefix from syscall names" the build has been broken for non-x86
architectures.

Remove the sys_ prefix from syscall names and related macros as has
already been done for x86.

Correct "OVVERRIDE" typo in arm and powerpc headers while we're at it.

Successfully built against v3.17 for 32- and 64-bit mips and powerpc;
runtime tested on arm, verifying that mmap2, execve, and clone syscall
events are captured.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>