sky130: add support for sky130_osu_sc_t18

Initial support for the t18 variant of OSU sky130.

Good enough to build the spm design from OpenLane, but requires a few
fixes in sky130_osu_sc_t18 and in OpenRoad.
diff --git a/sky130/magic/sky130.magicrc b/sky130/magic/sky130.magicrc
index db999db..7fe3735 100644
--- a/sky130/magic/sky130.magicrc
+++ b/sky130/magic/sky130.magicrc
@@ -60,6 +60,7 @@
     addpath ${PDKPATH}/libs.ref/${MAGTYPE}/sky130_fd_sc_ls
     addpath ${PDKPATH}/libs.ref/${MAGTYPE}/sky130_fd_sc_ms
     addpath ${PDKPATH}/libs.ref/${MAGTYPE}/sky130_osu_sc
+    addpath ${PDKPATH}/libs.ref/${MAGTYPE}/sky130_osu_sc_t18
 } else {
     addpath ${PDKPATH}/libs.ref/sky130_fd_pr/${MAGTYPE}
     addpath ${PDKPATH}/libs.ref/sky130_fd_io/${MAGTYPE}
@@ -71,6 +72,7 @@
     addpath ${PDKPATH}/libs.ref/sky130_fd_sc_ls/${MAGTYPE}
     addpath ${PDKPATH}/libs.ref/sky130_fd_sc_ms/${MAGTYPE}
     addpath ${PDKPATH}/libs.ref/sky130_osu_sc/${MAGTYPE}
+    addpath ${PDKPATH}/libs.ref/sky130_osu_sc_t18/${MAGTYPE}
 }
 
 # add path to GDS cells
@@ -86,6 +88,7 @@
     path cell +${PDKPATH}/libs.ref/gds/sky130_fd_sc_ls
     path cell +${PDKPATH}/libs.ref/gds/sky130_fd_sc_ms
     path cell +${PDKPATH}/libs.ref/gds/sky130_osu130
+    path cell +${PDKPATH}/libs.ref/gds/sky130_osu130_t18
 } else {
     path cell ${PDKPATH}/libs.ref/sky130_fd_pr/gds
     path cell +${PDKPATH}/libs.ref/sky130_fd_io/gds
@@ -97,6 +100,7 @@
     path cell +${PDKPATH}/libs.ref/sky130_fd_sc_ls/gds
     path cell +${PDKPATH}/libs.ref/sky130_fd_sc_ms/gds
     path cell +${PDKPATH}/libs.ref/sky130_osu130/gds
+    path cell +${PDKPATH}/libs.ref/sky130_osu130_t18/gds
 }
 #endif (FULLTECH)