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