[ulib][musl][ldso] Work around compiler-generated memset PLT calls

Early in the dynamic linker it's not safe to use exported symbols
in calls since those go via PLT. When the compiler generates a
call to memset, it will be via the PLT unless some symbol renaming
shenanigans are done to avoid it.

Test: CQ & manually tested arm64 LTO build
Change-Id: Ie9387bd457d7af8f53700d9016fed3a626f921c8

[ulib][musl] Optimized aarch64 string routines

These are taken verbatim from Linaro's cortex-strings package.
One mechanical cosmetic change to the assembly was necessary for
it to build with Clang/LLVM's assembler because the Linaro code
was using a GAS variant of the official syntax instead of what
the ARMv8 manual says to use. We can revert that to go back
to verbatim when https://bugs.llvm.org//show_bug.cgi?id=34488 is fixed.

MG-1111 #done

Change-Id: I186ee61b9184f247ba77a27622389eee4bde493c