Fix panics in api_level.go and apex.go when using riscv64 as the target
arch.
Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true nothing
Change-Id: I85c7685f3d14fa2dc7ffbcdea7f490feca304ef7
Add barebones riscv64-linux-android support. This should be enough
to add riscv64-specific entries to Android.bp files, but can't
actually compile anything until there are riscv64 toolchains.
Test: arch_test.go
Change-Id: I0dcc7e797d9352dd38243be908a7f19004ff3db1