linux-user: Fix parse_elf_properties GNU0_MAGIC check

linux-user: Fix parse_elf_properties GNU0_MAGIC check

Comparing a string of 4 bytes only works in little-endian.

Adjust bulk bswap to only apply to the note payload.
Perform swapping of the note header manually; the magic
is defined so that it does not need a runtime swap.

Fixes: 83f990eb5adb ("linux-user/elfload: Parse NT_GNU_PROPERTY_TYPE_0 notes")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2596
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>