x86: remap text/data/bss with appropriate permissions

x86: remap text/data/bss with appropriate permissions

c/s cf39362 "x86: use 2M superpages for text/data/bss mappings" served two
purposes; to map the primary code and data with appropriate pagetable
permissions (rather than unilaterally RWX), and to reduce the TLB pressure.

The extra alignment exposed a SYSLinux issue, and was partly reverted by c/s
0b8a172 "x86: partially revert use of 2M mappings for hypervisor image".

This change reinstates the pagetable permission improvements while avoiding
the 2M alignment issue.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>