[RISCV] Add -march=unset to cancel and ignore a previous -march. (#148321) -mcpu is used to determine the ISA string if an explicit -march is not present on the command line. If there is a -march present it always has priority over -mcpu regardless of where it appears in the command line. This can cause issues if -march appears in a Makefile and a user wants to override it with an -mcpu on the command line. The user would need to provide a potentially long ISA string to -march that matches the -mcpu in order to override the MakeFile. This issue can also be seen on Compiler Explorer where the rv64gc toolchain is passed -march=rv64gc before any user command line options are added. If you pass -mcpu=spacemit-x60, vectors will not be enabled due to the hidden -march. This patch adds a new option for -march, "unset" that makes the -march ignored for purposes of prioritizing over -mcpu. Now a user can write -march=unset -mcpu=<cpu_name>. This is also implemented by gcc for ARM32. An alternative would be to allow -march to take a cpu name, but that requires "-march=<cpu_name> -mcpu=<cpu_name>" or "-march=<cpu_name> -mtune=<cpu_name>" to ensure the tune cpu also gets updated. IMHO, needing to repeat the CPU name twice isn't friendly and invites mistakes. gcc has implemented this as well.
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