decodetree: Improve identifier matching

decodetree: Improve identifier matching

Only argument set members have to be C identifiers, everything
else gets prefixed during conversion to C.  Some places just
checked the leading character, and some places matched a leading
character plus a C identifier.

Convert everything to match full identifiers, including the
[&%@&] prefix, and drop the full C identifier requirement.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200903192334.1603773-1-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>