`-Zregparm=3` support in `rustc`

[ojeda]

i.e. the equivalent of Clang's/GCC's -mregparm=3. The kernel needs it to support the x86 32-bit architecture, together with -Zreg-struct-return.

It could potentially be a "global target feature", i.e. a target feature that is required to be set the same way for all compilation units.

https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html#index-regparm-function-attribute_002c-x86
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mregparm