meson.bbclass: avoid unexpected operating-system names

For example 'linux-gnueabi' is not what meson consumers expect: See canonical
table of OS names for Meson (thanks Ross). Surprisingly this did not pop up
earlier but wrong system-name can break building as described in [1]

[1] https://lists.openembedded.org/g/openembedded-core/topic/meson_host_machine_system/73023862



Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>