Add header file for XCValu

Created by: MaryBennett

Files Changed:

• gcc/config.gcc: Add header file.
• gcc/config/riscv/riscv_corev_alu.h: Add header file.
• gcc/testsuite/gcc.target/riscv/alu-c-api.c: New Test.

AP:

• Add test case for headers
• Locally test that headers work with find/ replace