[LLVM][inline-asm][Altmacor] Altmacro string delimiter '<..>'

In this patch, I introduce a new altmacro string delimiter. 
This review is the second review in a series of four reviews.
(one for each altmacro feature: LOCAL, string delimiter, string '!' escape sign and absolute expression as a string '%' ).

In the alternate macro mode, you can delimit strings with matching angle brackets <..> 
when using it as a part of calling macro arguments.

As described in the https://sourceware.org/binutils/docs-2.27/as/Altmacro.html
"<string>
You can delimit strings with matching angle brackets."

assumptions:

1. If an argument begins with '<' and ends with '>'. The argument is considered as a string.
2. Except adding new string mark '<..>', a regular macro behavior is expected.
3. The altmacro cannot affect the regular less/greater behavior.
4. If a comma is present inside an angle brackets it considered as a character and not as a separator.

Differential Revision: https://reviews.llvm.org/D32701

llvm-svn: 302135