Stop using grep -P.

This might let us get to using toybox grep for the build faster than if
we have to implement PCRE support.

Test: builds
Change-Id: Idaf24cfefbf18fdc442871f24fe4fe9b2152359a

tests/touch-obj-on-success

@@ -4,5 +4,8 @@
 # from that command line and touches it.
 
 "$@"
-obj="$(echo "$@" | grep -oP '\S+\.o\b')"
-touch "${obj}"
+for arg in "$@"; do
+  if [[ "$arg" == *.o ]]; then
+    touch "$arg"
+  fi
+done