gdb/alpha: Redefine fpcr with fpcr_flags type

gdb/alpha: Redefine fpcr with fpcr_flags type

This commit adds fpcr_flags and dyn_rm_enum types to define the fpcr.

For details on the floating-point control register (fpcr), please see
the Alpha Architecture Reference Manual, 4th Ed. [1]; in brief, it
consists of a 64-bit bitfield with most bits reserved/unused. All but a
pair of the used bits are boolean flags; the exception, DYN_RM, is a
2-bit enum indicating the IEEE rounding mode and is defined as a
dyn_rm_enum type in the target description annex.

[1] https://archive.org/details/dec-alpha_arch_ref

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Iea54c9e201faae6147a03de124b0368752bce060