linux-user/elfload: Write corefile elf header in one block

linux-user/elfload: Write corefile elf header in one block

Fixes a bug in which write_note() wrote namesz_rounded
and datasz_rounded bytes, even though name and data
pointers contain only the unrounded number of bytes.

Instead of many small writes, allocate a block to contain all
of the elf headers and all of the notes.  Copy the data into the
block piecemeal and the write it to the file as a chunk.
This also avoids the need to lseek forward for alignment.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>