LoongArch: Set default alignment for functions and labels with -mtune

LoongArch: Set default alignment for functions and labels with -mtune

The LA464 micro-architecture is sensitive to alignment of code.  The
Loongson team has benchmarked various combinations of function, the
results [1] show that 16-byte label alignment together with 32-byte
function alignment gives best results in terms of SPEC score.

Add a mtune-based table-driven mechanism to set the default of
-falign-{functions,labels}.  As LA464 is the first (and the only for
now) uarch supported by GCC, the same setting is also used for
the "generic" -mtune=loongarch64.  In the future we may set different
settings for LA{2,3,6}64 once we add the support for them.

Bootstrapped and regtested on loongarch64-linux-gnu.  Ok for trunk?

gcc/ChangeLog:

	* config/loongarch/loongarch-tune.h (loongarch_align): New
	struct.
	* config/loongarch/loongarch-def.h (loongarch_cpu_align): New
	array.
	* config/loongarch/loongarch-def.c (loongarch_cpu_align): Define
	the array.
	* config/loongarch/loongarch.cc
	(loongarch_option_override_internal): Set the value of
	-falign-functions= if -falign-functions is enabled but no value
	is given.  Likewise for -falign-labels=.