[arm] Don't generate invalid LDRD insns

[arm] Don't generate invalid LDRD insns

This fixes a bug in the arm backend where GCC generates invalid LDRD
instructions. The LDRD instruction requires the first transfer register to be
even, but GCC attempts to use odd registers here. For example, with the
following C code:

    struct c {
      double a;
    } __attribute((aligned)) __attribute((packed));
    struct c d;
    struct c f(struct c);
    void e() { f(d); }

The struct d is passed in registers r1 and r2 to the function f, and GCC
attempted to do this with a LDRD instruction when compiling with -march=armv7-a
on a soft float toolchain.

The fix is analogous to the corresponding one for STRD in the same function:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=52057dc4ac5295caebf83147f688d769c93cbc8d

2020-05-18  Alex Coplan  <alex.coplan@arm.com>

gcc/:
	* config/arm/arm.c (output_move_double): Fix codegen when loading into
	a register pair with an odd base register.

gcc/testsuite/:
	* gcc.c-torture/compile/packed-aligned-1.c: New test.
	* gcc.c-torture/execute/packed-aligned.c: New test.