Support multiple instructions per line separated by semicolons
Replace fadds with vadd.f32 for armasm

The previous replacement actually didn't work, which lead to
VFP not being enabled by the libav configure script.
Support temporary labels not starting at the start of the line
remove all lines starting with '#'

'#' at the start of the line indicates on all archs the start of a
comment or a preprocessor directive. This is after preprocessing so
removing the lines as comment is safe. Line number comments are stripped
by this too since gas-preprocessor.pl ignores line numbers. So it
removes just misleading debug information.

Incidentally fixes gas detection in libav's configure with Xcode on
aarch64. The temporary filename contains two '/'. The filename is added
as line number comment during preprocessing. The comment removal in
gas-preprocessor detects it on aarch64. Clang fails since the line
number comment has an unterminated string constant.
Pass -h through directly just as -v

This allows doing a simple no-op to test that gas-preprocessor
exists and works, since armasm doesn't have a -v option.
Undefine platform specific predefined defines when preprocessing C source for armasm

Since plain 'cpp' is used for preprocessing, it inherits whatever
platform specific predefined defines.

Previously this was set on the libav configure line, such as
--as='gas-preprocessor.pl -as-type armasm -- armasm -nologo -U__ELF__ -ignore 4509',
but if gas-preprocessor is to be enabled automatically, one shouldn't
need to pass -U__ELF__ via the --as flag. Also, if the option
is passed there, armasm detection fails, since "armasm -U__ELF__"
fails, since -U isn't a valid armasm option (gas-preprocessor
filters it out and only passes it to cpp).

This does break the currently unsupported setup of using the
original ARM armasm (contrary to the MS version) for building
assembly for linux though, where the __ELF__ define should be
kept. But there's not much need to support that use case, since
GNU as can be used just fine along with armcc.
Merge commit '6bbbfbed4b392a6ecf5cb3835d80d268855afdf2'

* commit '6bbbfbed4b392a6ecf5cb3835d80d268855afdf2':
  Undefine platform specific predefined defines when preprocessing C source for armasm
  Pass -h through directly just as -v
  remove all lines starting with '#'
  Support temporary labels not starting at the start of the line
  Replace fadds with vadd.f32 for armasm
  Support multiple instructions per line separated by semicolons