[feature] Allow modification of system/cpu_family/cpu/endian in generated MesonToolchain toolchain file

[madebr]

Hello,

I'm currently creating recipes+packages aimed for the Nintendo Switch.
I added Nintendo Switch to os in my settings.yml.

While creating a recipe for the mesa port, I'm using the MesonToolchain class.
This creates the following section:

[build_machine]

system = 'linux'
cpu_family = 'x86_64'
cpu = 'x86_64'
endian = 'little'

[host_machine]

system = 'nintendo switch'
cpu_family = 'aarch64'
cpu = 'armv8'
endian = 'little'

[target_machine]

system = 'nintendo switch'
cpu_family = 'aarch64'
cpu = 'armv8'
endian = 'little'

The correct data for nintendo should be:

[host_machine]
system = 'horizon'
cpu_family = 'aarch64'
cpu = 'armv8'
endian = 'little'

What I'm asking for is a feature to override these default/calculated values.

Right now, the toolchain method is the following:

    def toolchain(self):
        tc = MesonToolchain(self)
        tc.generate()

It would be useful to make this possible:

    def toolchain(self):
        tc = MesonToolchain(self)
        tc.host_machine.system = "horizon"
        tc.target_machine.system = "horizon"
        tc.generate()

Or something similar...

CC'ing @SSE4 for being the strong man behind MesonToolchain.

[SSE4]

it shouldn't be that hard to implement. however, I'd like to ensure we're aligned with interfaces (e.g. with AutoTools/CMake on how do they set custom GNU triple or CMAKE_SYSTEM_PROCESSOR).