toolchain: arcmwdt: linker: fix placeholders for arcmwdt toolchain

[IRISZZW]

syntax ". += length;" not work with arcmwdt toolchain, let's using
". = . + length;", which both work with gnu and arcmwdt toolchain.

Signed-off-by: Watson Zeng zhiwei@synopsys.com

Fix: #35479

[stephanosio]

Ideally, this patch should be accompanied by an automated check that prevents use of symbol f= expression, where f is any supported operator, in the linker scripts.

[IRISZZW]

Ideally, this patch should be accompanied by an automated check that prevents use of symbol f= expression, where f is any supported operator, in the linker scripts.

@stephanosio Is there any similar design that I can take a reference? And I think your ideal should be an enhancement, maybe we can add it in v2.7.0.

[stephanosio]

Is there any similar design that I can take a reference? And I think your ideal should be an enhancement, maybe we can add it in v2.7.0.

@IRISZZW Possibly, we can update checkpatch.pl to do just that in the future; or alternatively, make arcmwdt generate errors when an unsupported statement is encountered; or, even better, make arcmwdt support +=.

[ruuddw]

btw, I think it is not just the Metaware linker than doesn't like this, clang/llvm linker and others might not either.