Change the (internal to this file only) enum names from gcc_ to

ehframe_ & from gdb_ to stabs_ for clarity.

Also document the fact that i386 eh_frame on Darwin has the register
numbers swapped for ebp/esp from the DWARF register numbers so no one
copies these defines for other i386 ABI plugins.  This bug only ever
existed on Darwin.

No code changes, just renaming variables.

llvm-svn: 246834