x86emul: fix SYSCALL/SYSENTER/SYSEXIT emulation

x86emul: fix SYSCALL/SYSENTER/SYSEXIT emulation

SYSCALL:
- make sure SS selector has RPL 0
- only use 32 bits of RIP to fill RCX when target execution mode is 32-bit
- don't shadow function wide variable 'rc'
- consolidate CS attribute setting into single statements
- drop pointless initializers and casts
- drop redundant MSR_STAR read (as suggested by Andrew Cooper)

SYSENTER/SYSEXIT:
- #GP condition doesn't depend on guest mode
- only use 32 bits for setting RIP/RSP when target execution mode is 32-bit
- don't shadow function wide variable 'rc'
- consolidate CS attribute setting into single statements
- drop pointless (and inconsistently used) casts

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