Produce _alwayslink sibling targets through bp2build

Add Alwayslink to bazelPrebuiltLibraryStaticAttributes
Also adjust the corresponding tests to espect these new targets.

Test: m libc
Test: b build --config=android //prebuilts/clang/host/linux-x86:libunwind-exported_alwayslink
Test: bp2build.sh
Bug: 267214124
Change-Id: I4d996aef55321f1827947f00b0eaa52d0a14413e