linux-user: Emulate /proc/self/smaps

/proc/self/smaps is an extension of /proc/self/maps: it provides the
same lines, plus additional information about each range.

GDB uses /proc/self/smaps when available, which means that
generate-core-file tries it first before falling back to
/proc/self/maps. This, in turn, causes it to dump the host mappings,
since /proc/self/smaps is not emulated and is just passed through.

Fix by emulating /proc/self/smaps. Provide true values only for
Size, KernelPageSize, MMUPageSize and VmFlags. Leave all other values
at 0, which is a valid conservative estimate.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230621203627.1808446-4-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-34-alex.bennee@linaro.org>