gas/:

gas/:
* config/tc-mips.c (AT): Rename to...
(ATREG): ... this.
(AT): New definition.
(mips_set_options): Rename "noat" to "at"; change the type.
(mips_opts): Update accordingly.
(append_insn): Likewise.
(macro_build_ldst_constoffset): Likewise.
(load_address): Likewise.
(macro, macro2): Likewise.
(s_mipsset): Handle ".set at=REG".  Update handling of ".set at"
and ".set noat".

gas/testsuite/:
* gas/mips/at-1.d, gas/mips/at-2.l: New tests to check the ".set
at=REG" directive.
* gas/mips/at-1.s, gas/mips/at-2.s: Sources for the new tests.
* gas/mips/mips.exp: Run the new tests.