Fix Alpha native GDB build

[Commit log by Simon Marchi]

I get this error:

  CXX    linux-nat.o
/home/simark/src/binutils-gdb/gdb/linux-nat.c: In function 'void save_stop_reason(lwp_info*)':
/home/simark/src/binutils-gdb/gdb/linux-nat.c:2718:9: error: duplicated 'if' condition [-Werror=duplicated-cond]
    else if (GDB_ARCH_IS_TRAP_HWBKPT (siginfo.si_code))
         ^~
In file included from /home/simark/src/binutils-gdb/gdb/linux-nat.c:31:0:
/home/simark/src/binutils-gdb/gdb/nat/linux-ptrace.h:173:41: note: previously used here
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
                                    ~~~~~^~~~~~~~~~~~~~
/home/simark/src/binutils-gdb/gdb/linux-nat.c:2709:13: note: in expansion of macro 'GDB_ARCH_IS_TRAP_BRKPT'
    else if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code))
             ^~~~~~~~~~~~~~~~~~~~~~

For Alpha, we currently define GDB_ARCH_IS_TRAP_BRKPT and
GDB_ARCH_IS_TRAP_HWBKPT both to ((X) == TRAP_BRKPT), which causes the
two if branches to be duplicated.

Alpha doesn't have hardware breakpoints, so the Linux kernel for Alpha
never sets si_code to TRAP_HWBKPT.  We can just remove the special
definitions of these macros for __alpha__ and rely on the default ones.
Since the kernel will never report TRAP_HWBKPT, we will just never enter
the "hardware breakpoint" branch on Alpha (which is fine since it
doesn't have them).

gdb/ChangeLog:

	* nat/linux-ptrace.h [__alpha__]
	(GDB_ARCH_IS_TRAP_BRKPT, GDB_ARCH_IS_TRAP_HWBKPT): Remove
	definitions.