The most interesting or most complicated part here is the syscall_nr.h
generators.  In order to keep the generation logic all in meson.build,
I am adding to config_target the name of the .tbl file, and making the
generated file syscall<SUFFIX>_nr.h for input file syscall<SUFFIX>.tbl.

For architectures where the input file is not named syscall_nr.tbl,
syscall_nr.h has to be a source file; it's just a forwarder for x86
(i386/x86_64), while for MIPS64 it chooses between N32 and N64 ABIs.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>