Add bp2build support for cc_prebuilt_object

Add bp2build support for cc_prebuilt_object to allow buiiding
musl crt objects that use prebuilt clang_rt.crtbegin and
clang_rt.crtend.

Bug: 259266326
Test: //extenral/musl:libc_musl builds
Change-Id: Ic2b0375bc6e7336d31c9e2cf51bdc2ea894185aa
diff --git a/cc/cc.go b/cc/cc.go
index 2ff5bba..29bb84e 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -3775,7 +3775,9 @@
 			testBinaryBp2build(ctx, c)
 		}
 	case object:
-		if !prebuilt {
+		if prebuilt {
+			prebuiltObjectBp2Build(ctx, c)
+		} else {
 			objectBp2Build(ctx, c)
 		}
 	case fullLibrary: