efi/libstub: Avoid legacy decompressor zlib/zstd wrappers

Remove EFI zboot's dependency on the decompression wrappers used by the
legacy decompressor boot code, which can only process the input in one
go, and this will not work for upcoming support for embedded ELF images.
They also do some odd things like providing a barebones malloc()
implementation, which is not needed in a hosted environment such as the
EFI boot services.

So instead, implement GZIP deflate and ZSTD decompression in terms of
the underlying libraries. Support for other compression algoritms has
already been dropped.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>