libpmc: always generate libpmc_events.c

The jevents build tool will create an empty table if it doesn't find any
events, so we can remove the extra $MACHINE_CPUARCH checks.

Reviewed by:	gnn, ray, emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30531

Makefile.libcompat

@@ -51,7 +51,7 @@ _LC_INCDIRS=	\
 .if ${MK_FILE} != "no"
 _libmagic=	lib/libmagic
 .endif
-.if ${MK_PMC} != "no" && ${TARGET_ARCH} == "amd64"
+.if ${MK_PMC} != "no"
 _jevents=	lib/libpmc/pmu-events
 .endif