[lldb] [Process/FreeBSDRemote] Correct DS/ES/FS/GS register sizes

Fix DS/ES/FS/GS register sizes in getter/setter for FreeBSD.  Apparently
only CS and SS registers are specified as 64/32-bit in LLDB, while
the others are specified as 16-bit.  This fixes the failing
StandardStartupTest.TestStopReplyContainsThreadPcs lldb-server unittest.

Differential Revision: https://reviews.llvm.org/D91076