toolchain: gcc: Add a new LTO_SINGLE_THREADED option for LTO

[carlescufi]

As described in this issue:
zephyrproject-rtos/sdk-ng#1038 -flto=auto fails on some (slower) Windows machines due to an issue with the Zephyr SDK's GCC toolchain for Windows.
In order to allow users to work around this issue, introduce a new CONFIG_LTO_SINGLE_THREADED option that switches to -flto=1, which enforces a single thread when processing LTO.

[tejlmand]

Thanks for fixing this, but we need corresponding property names in the templates:
https://github.com/zephyrproject-rtos/zephyr/blob/main/cmake/compiler/compiler_flags_template.cmake
https://github.com/zephyrproject-rtos/zephyr/blob/main/cmake/linker/linker_flags_template.cmake

and it seems the existing optimization_lto and lto_arguments are also missing 🙁

[tejlmand]

@RobinKastberg will you take a look at corresponding IAR flags to use ?
@evgeniy-paltsev will you take a look at corresponding MetaWare flags to use ?

so that this property is not empty for those toolchains 🙂

[carlescufi]

Thanks for fixing this, but we need corresponding property names in the templates: https://github.com/zephyrproject-rtos/zephyr/blob/main/cmake/compiler/compiler_flags_template.cmake https://github.com/zephyrproject-rtos/zephyr/blob/main/cmake/linker/linker_flags_template.cmake

and it seems the existing optimization_lto and lto_arguments are also missing 🙁

Thanks, done @tejlmand.

[carlescufi]

@RobinKastberg will you take a look at corresponding IAR flags to use ?
@evgeniy-paltsev will you take a look at corresponding MetaWare flags to use ?

@tejlmand I don't think LTO was supported on these toolchains to start with, so this new single-threaded option is not applicable to those until they actually support LTO itself?

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[RobinKastberg]

@RobinKastberg will you take a look at corresponding IAR flags to use ?
@evgeniy-paltsev will you take a look at corresponding MetaWare flags to use ?

@tejlmand I don't think LTO was supported on these toolchains to start with, so this new single-threaded option is not applicable to those until they actually support LTO itself?

Yeah... This is 100% the way for us. But I appreciate being asked anyways! You can leave it empty

[tejlmand]

@tejlmand I don't think LTO was supported on these toolchains to start with, so this new single-threaded option is not applicable to those until they actually support LTO itself?

Seems like it.
@carlescufi note, the comment for @RobinKastberg and @evgeniy-paltsev is not blocking for this PR but was for them to be aware and follow-up with similar work for their toolchains (hence the reason the comment was made independent of the request-for-changes review comment 🙂 )