Some tests are only applicable to specific ISAs. This can be indicated
with an ISA specification:
test legalizer
isa riscv
function foo() {
....
}
The ISA specifications have the same format as the test lines: The name
of the ISA following by optional settings until the end of the line.
Also parse `set` commands mixed in with the `isa` commands. These are
used to set ISA-independent settings as defined in
meta/cretonne/settings.py.
