meson: Fix Meson CPU names for powerpc CPUs

Nixpkgs expects the `builtin.currentSystem` for POWER CPUs to be:

`powerpc[64][le]-linux`

But using `host_machine.cpu_family()` for the CPU part of the system
string on POWER produces this instead:

`ppc[64]-linux`

So evaluating Nixpkgs errors out on:

`error: Unknown CPU type: ppc64`

To fix this, change
`ppc` -> `powerpc`
`ppc64` -> `powerpc64`
and append `le` if `host_machine.endian() == 'little'`.

I can't actually test this on hardware rn due to hitting a kernel bug
on the host system when linking big things[1], but the approach here
is similar to how it was fixed in cppnix[2][3], so it *should* be fine.

[1] https://git.adelielinux.org/adelie/packages/-/issues/1315
[2] https://github.com/NixOS/nix/pull/13514
[3] https://github.com/NixOS/nix/pull/13520

Change-Id: Ib82839cdaf2198bf18b89e82caaa1217f88e11ed

meson.build

@@ -208,6 +208,16 @@ elif host_cpu == 'amd64'
   host_cpu = 'x86_64'
 elif host_cpu in ['armv6', 'armv7']
   host_cpu += 'l'
+elif host_cpu == 'ppc'
+  host_cpu = 'powerpc'
+  if host_machine.endian() == 'little'
+    host_cpu += 'le'
+  endif
+elif host_cpu == 'ppc64'
+  host_cpu = 'powerpc64'
+  if host_machine.endian() == 'little'
+    host_cpu += 'le'
+  endif
 endif
 
 host_system = host_machine.cpu_family() + '-' + host_machine.system()