fix: gnu triplet for linux x86

[ErniGH]

Changelog: Fix: Fix getting the gnu triplet for Linux x86.
Docs: Omit

Close #10211

Changed the gnu triplet for linux x86 systems from "x86-linux-gnu" to "i686-linux-gnu".

[jcar87]

For added context - the 32-bit Intel triplet in more recent times has been i686-linux-gnu - that is typically used by the distros and is the prefix of the 32-bit intel gcc toolchain.

x86-linux-gnu is still a valid triplet as per config.sub, and for the most part, when cross-building with Conan, autotools is fine with x86-linux-gnu. We have only seen a couple of issues:

• When cross-building a recipe from Linux/x86_64 to Linux/x86 - if the compiler is not specified in the profile (with CC/CXX en vars, or the new compiler executables config) - Autotools will try to locate the compiler based on the host prefix. if a valid toolchain is installed on the system, it will not be found if the triplet was x86-linux-gnu. It's still unlikely to have a profile like that, because the name of the compiler executables are going to be needed for other tools (like CMake)
• As reported in [bug] gnu triplet for os=Linux arch=x86 is suboptimal #10211 - some Autotools scripts (like https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=m4/stack-direction.m4;h=b33920f2885ae7aeb83e6f6ccaaf30b0e76c4e43;hb=f8eed11b15e9141d061900e4068ea1f3ba9b63f6) - may have some assumptions based on the architecture part of the triplet, and in this case, on line 37 it is clear that 32-bit intel is assumed to be i*86 with * being a number. Thus further evidencing that we should stay with convention.