linux-user: Correctly start brk after executable

info->brk was erroneously set to the end of highest addressed
writable segment which could result it in overlapping the executable.

As per load_elf_binary in fs/binfmt_elf.c in Linux, it should be
set to end of highest addressed segment.

Signed-off-by: Timothy E Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200728224615.326675-1-T.E.Baldwin99@members.leeds.ac.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>