KVM: selftests: Implement memcmp(), memcpy(), and memset() for guest use

Implement memcmp(), memcpy(), and memset() to override the compiler's
built-in versions in order to guarantee that the compiler won't generate
out-of-line calls to external functions via the PLT.  This allows the
helpers to be safely used in guest code, as KVM selftests don't support
dynamic loading of guest code.

Steal the implementations from the kernel's generic versions, sans the
optimizations in memcmp() for unaligned accesses.

Put the utilities in a separate compilation unit and build with
-ffreestanding to fudge around a gcc "feature" where it will optimize
memset(), memcpy(), etc... by generating a recursive call.  I.e. the
compiler optimizes itself into infinite recursion.  Alternatively, the
individual functions could be tagged with
optimize("no-tree-loop-distribute-patterns"), but using "optimize" for
anything but debug is discouraged, and Linus NAK'd the use of the flag
in the kernel proper[*].

https://lore.kernel.org/lkml/CAHk-=wik-oXnUpfZ6Hw37uLykc-_P0Apyn2XuX-odh-3Nzop8w@mail.gmail.com

Cc: Andrew Jones <andrew.jones@linux.dev>
Cc: Anup Patel <anup@brainfault.org>
Cc: Atish Patra <atishp@atishpatra.org>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220928233652.783504-2-seanjc@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>