Add support for the ASCII directive inside linker scripts.

 * ldlex.l: Add ASCII token.
 * ldgram.y: Add parsing of the ASCII command.
 * ldlang.c (lang_add_string): Add maximum size parameter.  Move escape character handling code into separate function.
 * ldlang.h (lang_add_string): Update prototype.
 * NEWS: Mention the new feature.
 * ld.texi (Output Section Data): Document the new directives.
 * testsuite/ld-scripts/asciz.t: Adjust to work on more architectures and to test more aspects of the ASCIZ directive.
 * testsuite/ld-scripts/asciz.d: Adjust to match the changes to the test linker script.
 * testsuite/ld-scripts/ascii.d: New test driver.
 * testsuite/ld-scripts/ascii.s: New test assembler source.
 * testsuite/ld-scripts/ascii.t: New test script.
 * testsuite/ld-scripts/script.exp: Run the new test.