[nvptx] Add march-map

Say we have an sm_50 board, and we want to run a benchmark using the highest
possible march setting.

Currently there's march=sm_30, march=sm_35, march=sm_53, but no march=sm_50.

So, we'd need to pick march=sm_35.

Likewise, for a test script that handles multiple boards, we'd need a mapping
from native board sm_xx to march, which might have to be updated with newer
gcc releases.

Add an option march-map, such that we can just specify march-map=sm_50, and
let the compiler map this to the appropriate march.

The option is implemented as a list of aliases, such that we have a somewhat
lengthy (17 lines in total):
...
$ gcc --help=target
  ...
  -march-map=sm_30            Same as -misa=sm_30.
  -march-map=sm_32            Same as -misa=sm_30.
  ...
  -march-map=sm_87            Same as -misa=sm_80.
  -march-map=sm_90            Same as -misa=sm_80.
...

This implementation was chosen in the hope that it'll be easier if
we end up with some misa multilib.

It would be nice to have the mapping list generated from an updated
nvptx-sm.def, but for now it's spelled out in nvptx.opt.

Tested on nvptx.

gcc/ChangeLog:

2022-03-29  Tom de Vries  <tdevries@suse.de>

	PR target/104714
	* config/nvptx/nvptx.opt (march-map=*): Add aliases.

gcc/testsuite/ChangeLog:

2022-03-29  Tom de Vries  <tdevries@suse.de>

	PR target/104714
	* gcc.target/nvptx/march-map.c: New test.