main/gcc: align default LIBRARY_PATH on riscv64 with existing arches

By modifying the relevant parts of the multilib configuration and startfile_prefix_spec we can continue avoiding the use of lib32/lib64 directories in conjunction with musl. With this applied the default LIBRARY_PATH on riscv64 becomes:

LIBRARY_PATH=/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/:/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/../../../../riscv64-alpine-linux-musl/lib/../lib/:/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/../../../../riscv64-alpine-linux-musl/lib/:/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/../../../:/lib/:/usr/lib/

The previous LIBRARY_PATH value was:

LIBRARY_PATH=/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/:/usr/lib/gcc/riscv64-alpine-linux-musl/11.2.1/../../../../riscv64-alpine-linux-musl/lib/:/lib/:/usr/lib/

which had a few issues (e.g. preferred /lib over /usr/lib which we don't do on other architectures).

Fixes #13369 (closed)