Implement -fpatchable-function-entry flag

[xoxorwr]

Context: https://forum.dlang.org/post/110h05u$2tc6$1@digitalmars.com

This PR implements the -fpatchable-function-entry=total_nops[,prefix_nops] compiler flag to enable NOP padding for function entries.

It provides the same effect as the GCC/Clang/Rust -fpatchable-function-entry flag.

It facilitates atomic hotpatching and hot reloading scenarios.

https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fpatchable-function-entry

https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fpatchable-function-entry

[dlang-bot]

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Testing this PR locally

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dub run digger -- build "master + dmd#23251"

[xoxorwr]

cc @rikkimax

[rikkimax]

I am aware that Microsoft uses this technique quite heavily in their products, including Windows.

I am also very hesitant to call this a good direction for D. There are some very difficult problems to solve in your -watch proposal, and I'd like to see how that proceeds before suggesting that this is a good feature for this purpose.

[xoxorwr]

Separate from the compiler daemon idea, this is a generally useful feature that could benefit a variety of tools.

In situations where you'd normally fall back to C (or even Rust), D becomes a viable option again.

[ibuclaw]

Put this in dmdparams as it's not used by the front-end.