gdb/linux-tdep: make read_mapping return a structure

gdb/linux-tdep: make read_mapping return a structure

Change read_mapping to return a structure instead of taking many output
parameters.  Change the string + length output parameters (permissions
and device) to be gdb::string_view, since that's what string_view is
for (a non-NULL terminated view on a string).  No changes in behavior
expected.

Change-Id: I86e627d84d3dda8c9b835592b0f4de8d90d12112