Add native Linux support for the PowerPC E500.

Add native Linux support for the PowerPC E500.
* ppc-tdep.h (struct gdbarch_tdep): New member: 'ppc_gprs_pseudo_p'.
* rs6000-tdep.c (rs6000_gdbarch_init): Initialize it to false on
all architectures except the E500.
* ppc-linux-nat.c: (PTRACE_GETEVRREGS, PTRACE_SETEVRREGS): New
#definitions.
(struct gdb_evrregset_t): New type.
(have_ptrace_getsetevrregs): New variable.
(get_spe_registers, read_spliced_spe_reg, fetch_spe_register,
fetch_spe_registers): New functions.
(fetch_register): Call fetch_spe_register as appropriate.
Assert that we're only passed raw register numbers.
(fetch_ppc_registers): Call fetch_spe_registers as appropriate.
Don't fetch gprs if they're pseudoregisters.
(set_spe_registers, write_spliced_spe_reg, store_spe_register,
store_spe_registers): New functions.
(store_register): Call store_spe_register as appropriate.
Assert that we're only passed raw register numbers.
(store_ppc_registers): Call store_spe_registers as appropriate.
Don't store gprs if they're pseudoregisters.