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