AArch64: don't override march to assembler with mcpu if march is specified [PR110901]

AArch64: don't override march to assembler with mcpu if march is specified [PR110901]

When both -mcpu and -march are specified, the value of -march wins out.

This is done correctly for the calls to cc1 and for the assembler directives we
put out in assembly files.

However in the call to as we don't do this and instead use the arch from the
cpu.  This leads to a situation that GCC cannot reliably be used to compile
assembly files which don't have a .arch directive.

This is quite common with .S files which use macros to selectively enable
codepath based on what the preprocessor sees.

The fix is to change MCPU_TO_MARCH_SPEC to not override the march if an march
is already specified.

gcc/ChangeLog:

	PR target/110901
	* config/aarch64/aarch64.h (MCPU_TO_MARCH_SPEC): Don't override if
	march is set.

gcc/testsuite/ChangeLog:

	PR target/110901
	* gcc.target/aarch64/options_set_29.c: New test.