build: do not hardcode AFLAGS for as-insn tests

build: do not hardcode AFLAGS for as-insn tests

Hardcoding as-insn to use AFLAGS is not correct. For once the test is
performed using a C file with inline assembly, and secondly the flags
used can be passed by the caller together with the CC.

Fix as-insn-check to pass the flags given as parameter to the test.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
[Fix usage comments as they are changing]
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>