Testsuite: Allow multiple lines of "as" in testsuite.

Committed on behalf of Matthew Malcomson.

This allows checking the command line parsing more easily than before by
allowing many command line invokations from the same .d file.

Each line is used as a set of flags, and the tests are ran against the output
of the assembler with each set.
Each line of assembler is treated as another set of tests (as if the test file
were copied to another with a different #as: line).

This patch includes some example uses where multiple testcases can be merged
into one file using this new functionality.

binutils/ChangeLog:

	* testsuite/lib/binutils-common.exp: Allow multiple "as" lines.

gas/ChangeLog:

	* testsuite/gas/aarch64/dotproduct.d: Use multiple "as" lines.
	* testsuite/gas/aarch64/dotproduct_armv8_4.d: Remove.
	* testsuite/gas/aarch64/dotproduct_armv8_4.s: Remove.
	* testsuite/gas/aarch64/illegal-dotproduct.d: Use multiple "as"
	lines.
	* testsuite/gas/aarch64/ldst-rcpc-armv8_2.d: Remove.
	* testsuite/gas/aarch64/ldst-rcpc.d: Use multiple "as" lines.