abi.h: Add LoongArch64 support

LoongArch is a new instruction set of Loongson 3A5000 CPU, currently in the development of the system, has completed the Loongarch 64bit version based on Debian 10.

We have started to submit patches to the community, such as binutils, autotools, meson, etc., and hope that kwiml can merge this patch as soon as possible, thank you.

More info about this patch:

$ uname -m
loongarch64
$ gcc -E -dM - < /dev/null|grep __loongarch64
#define __loongarch64 1
$ gcc -E -dM - < /dev/null|grep BYTE_ORDER
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__

Topic-rename: abi-la64

mentioned in merge request cmake/cmake!6028 (closed)

The test results:

$ make test
Running tests...
Test project /home/loongson/kwiml/_build
    Start 1: kwiml.test
1/1 Test #1: kwiml.test .......................   Passed    0.01 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.05 sec

changed the description

assigned to @brad.king

merged

mentioned in merge request cmake/cmake!6032 (merged)

mentioned in merge request vtk/vtk!8184 (closed)