[nvptx] Use .alias directive for mptx >= 6.3

Starting with ptx isa version 6.3, a ptx directive .alias is available.
Use this directive to support symbol aliases, as far as possible.

The alias support is off by default.  It can be turned on using a switch
-malias.

Furthermore, for pre-sm_75, it's not effective unless the ptx version is
bumped to 6.3 or higher using -mptx (given that the default for pre-sm_75 is
6.0).

The alias support has the following limitations.

Only function aliases are supported.

Weak aliases are not supported.  That is, if I disable the check in
nvptx_asm_output_def_from_decls that disallows this, a weak alias is emitted
and parsed by the driver.  But the test gcc.dg/globalalias.c starts failing,
with the behaviour matching the comment about "weird behavior of AIX's .set
pseudo-op": a weak alias may resolve to different functions in different
files.

Aliases to weak symbols are not supported (see gcc.dg/localalias.c).  This is
currently not prohibited by the compiler, but with the driver link we run
into: "error: Function test with .weak scope cannot be aliased".

Aliases to aliases are not supported (see libgomp.c-c++-common/pr96390.c).
This is currently not prohibited by the compiler, but with the driver link we
run into:  "Internal error: alias to unknown symbol" .

Unreferenced aliases are not emitted (these can occur f.i. when inlining a
call to an alias).  This avoids driver link error "Internal error: reference
to deleted section".

When enabling malias by default, libgomp detects alias support and
consequently libgomp.a will contains a few uses of .alias.  This however
results in aforementioned "Internal error: reference to deleted section" in
many test-cases.  Either there's some error with how .alias is used, or
there's a driver bug.  While this issue is not resolved, we keep malias
off-by-default.

At some point we may add support in the nvptx-tools linker for symbol
aliases, and define f.i. malias=ptx and malias=ld to choose between the two in
the compiler.

An example of where this support is useful, is the OvO (OpenMP vs Offload)
testsuite.  The testsuite passes already at -O2.  But at -O0, there are errors
in some c++ test-cases due to missing symbol alias support.  By compiling with
-malias, the whole testsuite passes also at -O0.

This patch causes a regression:
...
-PASS: gcc.dg/pr60797.c  (test for errors, line 4)
+FAIL: gcc.dg/pr60797.c  (test for errors, line 4)
...
The test-case is skipped for effective target alias, and both without and with
this patch the nvptx target is considered to not support it, so the test-case is
executed.  The test-case expects an error message along the lines of "alias
definitions not supported in this configuration", but instead we run into:
...
gcc.dg/pr60797.c:4:12: error: foo aliased to undefined symbol
...
This is probably due to the fact that the nvptx backend now defines macros
ASM_OUTPUT_DEF and ASM_OUTPUT_DEF_FROM_DECLS, so from the point of view of the
common part of the compiler, aliases are supported.

gcc/ChangeLog:

2022-03-18  Tom de Vries  <tdevries@suse.de>

	PR target/104957
	* config/nvptx/nvptx-protos.h (nvptx_asm_output_def_from_decls): Declare.
	* config/nvptx/nvptx.cc (write_fn_proto_1): Don't add function marker
	for alias.
	(SET_ASM_OP, NVPTX_ASM_OUTPUT_DEF): New macro def.
	(nvptx_asm_output_def_from_decls): New function.
	* config/nvptx/nvptx.h (ASM_OUTPUT_DEF): New macro def, define to
	gcc_unreachable ().
	(ASM_OUTPUT_DEF_FROM_DECLS): New macro def, define to
	nvptx_asm_output_def_from_decls.
	* config/nvptx/nvptx.opt (malias): New opt.

gcc/testsuite/ChangeLog:

2022-03-18  Tom de Vries  <tdevries@suse.de>

	PR target/104957
	* gcc.target/nvptx/alias-1.c: New test.
	* gcc.target/nvptx/alias-2.c: New test.
	* gcc.target/nvptx/alias-3.c: New test.
	* gcc.target/nvptx/alias-4.c: New test.
	* gcc.target/nvptx/nvptx.exp
	(check_effective_target_runtime_ptx_isa_version_6_3): New proc.